udf: Fix crash during mount

Fix a crash during an attempt to mount a filesystem that has both
Unallocated Space Table and Unallocated Space Bitmap. Such filesystem
actually violates the UDF standard so we just have to properly detect
such situation and refuse to mount such filesystem read-write. When we
are at it, verify also other constraints on the allocation information
mandated by the standard.

Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>