filter: Add support for the csum action in u32

[niax]

This patch adds support for the U32 action that can force a csum
recalculation in different layers of a packet. It closes #668

I wrote a small test program to match the behaviour of the equivalent tc invocation:

package main

import (
        "fmt"
        "github.com/vishvananda/netlink"
)

func main() {
        handle, err := netlink.NewHandle()
        if err != nil {
                panic(err)
        }

        link, err := handle.LinkByName("dummy0")
        if err != nil {
                panic(err)
        }

        // Key matches IPv6 packets
        u32_key_ipv6 :=
                netlink.TcU32Key{
                        Mask:    0x0000FF00,
                        Val:     0x00000600,
                        Off:     4,
                        OffMask: 0,
                }

        filter := netlink.U32{
                Actions: []netlink.Action{
                        &netlink.CsumAction{
                                ActionAttrs: netlink.ActionAttrs{
                                        Action: netlink.TC_ACT_UNSPEC,
                                },
                                UpdateFlags: netlink.TCA_CSUM_UPDATE_FLAG_TCP | netlink.TCA_CSUM_UPDATE_FLAG_UDP,
                        },
                },
                Sel: &netlink.TcU32Sel{
                        Keys: []netlink.TcU32Key{u32_key_ipv6},
                        Flags: netlink.TC_U32_TERMINAL,
                },
        }
        filter.LinkIndex = link.Attrs().Index
        filter.Parent = netlink.HANDLE_MIN_EGRESS
        filter.Priority = 999
        filter.Protocol = 3
        err = handle.FilterAdd(&filter)
        fmt.Printf("Errors: %+v\n", err)
}

This does the same as tc filter add dev dummy0 egress pref 200 u32 match ip6 protocol 6 0xff action csum tcp udp continue, which can be seen with a show:

$ tc filter show dev dummy0 egress
filter protocol all pref 200 u32 chain 0
filter protocol all pref 200 u32 chain 0 fh 800: ht divisor 1
filter protocol all pref 200 u32 chain 0 fh 800::800 order 2048 key ht 800 bkt 0 terminal flowid ??? not_in_hw
  match 00000600/0000ff00 at 4
        action order 1: csum (tcp, udp) action continue
        index 3 ref 1 bind 1

filter protocol all pref 999 u32 chain 0
filter protocol all pref 999 u32 chain 0 fh 801: ht divisor 1
filter protocol all pref 999 u32 chain 0 fh 801::800 order 2048 key ht 801 bkt 0 terminal flowid ??? not_in_hw
  match 00000600/0000ff00 at 4
        action order 1: csum (tcp, udp) action continue
        index 1 ref 1 bind 1

[niax]

@vishvananda (or someone else with perms to merge), is there something extra I'm missing to get this looked at?

[vishvananda]

This lgtm. I'll give @aboch a chance to comment on it before i merge

[niax]

@aboch any comments on this, or can @vishvananda merge?

[aboch]

LGTM