if_vlan: handle VID conflicts

[kp]

Summary

If we fail to change the vlan id we have to undo the removal (and vlan id
change) in the error path. Otherwise we'll have removed the vlan object from the
hash table, and have the wrong vlan id as well. Subsequent modification attempts
will then try to remove an entry which doesn't exist, and panic.

Undo the vlan id modification if the insertion in the hash table fails, and
re-insert it under the original vlan id.

PR: 279195
MFC atfer: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

[zlei]

Looks good to me.

sys/net/if_vlan.c

1727

Shall we assert check the return value of the re-inserting ? That should never fail.

[kp]

sys/net/if_vlan.c

1727

Yeah, good idea. I'll do that and push.

[zlei]

tests/sys/net/if_vlan.sh

318

${epair}a is not used by this test case, I think this line can be removed, only to reduce a little time for running tests...