btrfs: fix for write_super_lockfs/unlockfs error handling

Commit c4be0c1d

 added the ability for
write_super_lockfs to return errors, and renamed them to match.  But
btrfs didn't get converted.

Do the minimal conversion to make it compile again.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>