Fix double unlock if the source file disappears

[trasz]

Summary

Fix double unlock if the source file disappears; we would
unlock it here, only to unlock it again below, just before "bad".

[kib]

sys/fs/msdosfs/msdosfs_vnops.c

1144

So why this vrele() uses a_vfp pointer instead of fvp ? Is it possible that they are different (doubt it). If so wouldn't it be more natural to stop these intermediate vrele/unlocks, replacing them by final vput(fvp) ?

[trasz]

sys/fs/msdosfs/msdosfs_vnops.c

1144

It is possible they are different - this code happens after a call to relookup(), which can change fvp. In fact, it has to be different, given the conditional ("xp != ip").

[trasz]

(Tinderboxed.)