UDF: handle wrong superblock better

If UDF superblock is incorrect, we can fail to find a table of free /
allocated space and consequently Oops.  Handle this situation more
gracefully by ignoring the broken UDF partition.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>