Optimize query for unscanned files

[zero0cool0]

If my interpretation of the code in BackgroundScanner is correct, getNodeForFile() relies on the existence of an existing mount to determine the Node for a given fileId.

Sometimes, this fails. Specifically, this line is returning an empty array.
$cachedMounts = $this->userMountCache->getMountsForFileId($fileId)

Tracking this further, I found that no corresponding mount can be found for some fileId that are present in oc_filecache when joining oc_mounts through oc_storage.

Why is this even a problem, other than creating many log entries?

If the number of such entries in oc_filecache is large then the entire batch contains non-scannable entries, starving the scanning of other entries in oc_filecache that have a corresponding mount.

My PR proposes to only process files whose fileId that can be related to an existing mount.

More details:
I have a lot of entries in oc_filecache that look like this:

MariaDB [nextcloud]> select fileid,storage,path,size from oc_filecache where fileid=1044856;
+---------+---------+---------------------------------------------------------------+------+
| fileid  | storage | path                                                          | size |
+---------+---------+---------------------------------------------------------------+------+
| 1044856 |      30 | appdata_ocp617nvaw61/preview/2/7/b/3/b/c/9/667010/256-154.png | 7476 |
+---------+---------+---------------------------------------------------------------+------+

These entries correctly relate to an entry in oc_storages:

+------------+----------------------------+-----------+--------------+
| numeric_id | id                         | available | last_checked |
+------------+----------------------------+-----------+--------------+
|         30 | local::/var/www/html/data/ |         1 |         NULL |
+------------+----------------------------+-----------+--------------+

However, no mount can be found:

MariaDB [nextcloud]> select * from oc_mounts where storage_id=30;
Empty set (0.000 sec)

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