udf: fix directory offset handling

Patch cleaning up UDF directory offset handling missed modifications in dir.c
(because I've submitted an old version :(). Fix it.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Tested-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>