[MASTER] Port MTU configuration is different between DB and kernel

[tim-rj]

Description
When setting the port MTU, if it is less than 68, the APPL_DB data and the kernel MTU data will be inconsistent. For example, when it is set to 1, the APPL_DB data is 1, ASIC_DB data and sdk data is 23，but the kernel remains unchanged.

Steps to reproduce the issue:
1.set a port's MTU less than 68
2.view port status
3.view kernel status
4.view APPL_DB,ASIC_DB,sdk data
Describe the results you received:
When the port MTU is set to be less than 68, the MTU values of APPL_DB、PORT STATUS and the kernel are inconsistent
//Firstly, SET Ethernet19 to 1

root@sonic:/home/admin# config interface mtu Ethernet19 1

//view port status MTU

root@sonic:/home/admin# show interfaces status
  Interface            Lanes    Speed    MTU    FEC               Alias    Vlan    Oper    Admin            Type    Asym PFC
-----------  ---------------  -------  -----  -----  ------------------  ------  ------  -------  --------------  ----------
 Ethernet19               31      25G      1    N/A  twentyfiveGigE0/19  routed      up       up  SFP/SFP+/SFP28         off -----------port status is 1

//view APPL_DB,ASIC_DB,sdk data

root@sonic:/home/admin# redis-cli
127.0.0.1:6379> HGETALL "PORT_TABLE:Ethernet19"
11) "mtu"
12) "1"                        ------------appl_db is 1
127.0.0.1:6379[4]> SELECT 1
OK
127.0.0.1:6379[1]> HGETALL "ASIC_STATE:SAI_OBJECT_TYPE_PORT:oid:0x1000000000014"
7) "SAI_PORT_ATTR_MTU"
8) "23"  --------------------------------> ASCI_DB is 23
127.0.0.1:6379[1]> exit

root@sonic:/home/admin# bcmsh 
Press Enter to show prompt.
Press Ctrl+C to exit.
NOTICE: Only one bcmsh or bcmcmd can connect to the shell at same time.

drivshell>ps
ps
                 ena/        speed/ link auto    STP                  lrn  inter   max   cut   loop
           port  link  Lns   duplex scan neg?   state   pause  discrd ops   face frame  thru?  back
      xe18( 31)  up     1   25G  FD   SW  No   Forward          None    F     CR    23    No      ------------SDK is 23
drivshell>root@sonic:/home/admin# timed out waiting for input: auto-logout
admin@sonic:~$ 

//view kernel status

root@sonic:/home/admin# ifconfig Ethernet19
Ethernet19: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9100-------------->kernel is 9100
        inet 10.0.0.38  netmask 255.255.255.254  broadcast 0.0.0.0
        inet6 fe80::c2b8:e6ff:fec0:abb9  prefixlen 64  scopeid 0x20<link>
        ether c0:b8:e6:c0:ab:b9  txqueuelen 1000  (Ethernet)
        RX packets 344  bytes 49457 (48.2 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 442  bytes 75870 (74.0 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Describe the results you expected:
The kernel and port status be the same, or when I enter a value less than 68, it should tell me that it is an invalid value
Additional information you deem important (e.g. issue happens only occasionally):

root@sonic:/home/admin# show version 

SONiC Software Version: SONiC.master.521-dirty-20201214.053854
Distribution: Debian 10.7
Kernel: 4.19.0-9-2-amd64
Build commit: e010d83f
Build date: Mon Dec 14 05:46:03 UTC 2020
Built by: johnar@jenkins-worker-8

Platform: x86_64-ruijie_b6510-48vs8cq-r0
HwSKU: B6510-48VS8CQ
ASIC: broadcom
ASIC Count: 1
Serial Number: G1PW94A000097
Uptime: 08:22:44 up 15 min,  3 users,  load average: 0.88, 0.68, 0.52

Docker images:
REPOSITORY                    TAG                                IMAGE ID            SIZE
docker-snmp                   latest                             0918c47c004b        485MB
docker-snmp                   master.521-dirty-20201214.053854   0918c47c004b        485MB
docker-teamd                  latest                             8b64452eaccf        493MB
docker-teamd                  master.521-dirty-20201214.053854   8b64452eaccf        493MB
docker-sonic-mgmt-framework   latest                             e69447f1eca3        610MB
docker-sonic-mgmt-framework   master.521-dirty-20201214.053854   e69447f1eca3        610MB
docker-router-advertiser      latest                             436769004c67        449MB
docker-router-advertiser      master.521-dirty-20201214.053854   436769004c67        449MB
docker-platform-monitor       latest                             e9df204fae71        574MB
docker-platform-monitor       master.521-dirty-20201214.053854   e9df204fae71        574MB
docker-lldp                   latest                             5c6bd28fcb98        489MB
docker-lldp                   master.521-dirty-20201214.053854   5c6bd28fcb98        489MB
docker-dhcp-relay             latest                             56f9ec408253        456MB
docker-dhcp-relay             master.521-dirty-20201214.053854   56f9ec408253        456MB
docker-database               latest                             f17f457e082d        449MB
docker-database               master.521-dirty-20201214.053854   f17f457e082d        449MB
docker-orchagent              latest                             59d41bf1b917        507MB
docker-orchagent              master.521-dirty-20201214.053854   59d41bf1b917        507MB
docker-nat                    latest                             b53fff4428a5        496MB
docker-nat                    master.521-dirty-20201214.053854   b53fff4428a5        496MB
docker-sonic-telemetry        latest                             810b6f0cdc1a        524MB
docker-sonic-telemetry        master.521-dirty-20201214.053854   810b6f0cdc1a        524MB
docker-fpm-frr                latest                             156874d313ad        508MB
docker-fpm-frr                master.521-dirty-20201214.053854   156874d313ad        508MB
docker-sflow                  latest                             f5c9dad286ac        493MB
docker-sflow                  master.521-dirty-20201214.053854   f5c9dad286ac        493MB
docker-syncd-brcm             latest                             4bcc015ebd06        542MB
docker-syncd-brcm             master.521-dirty-20201214.053854   4bcc015ebd06        542MB

root@sonic:/home/admin#