udf: Fix mounting of Win7 created UDF filesystems

Win7 is creating UDF filesystems with single partition with number 8192.
Current partition descriptor scanning code does not handle this well as
it incorrectly assumes that partition numbers will form mostly contiguous
space of small numbers. This results in unmountable media due to errors
like:

UDF-fs: error (device dm-1): udf_read_tagged: tag version 0x0000 != 0x0002 || 0x0003, block 0
UDF-fs: warning (device dm-1): udf_fill_super: No fileset found

Fix the problem by handling partition descriptors in a way that sparse
partition numbering does not matter.

Reported-and-tested-by: jean-luc malet <jeanluc.malet@gmail.com>
CC: stable@vger.kernel.org
Fixes: 7b78fd02
Signed-off-by: Jan Kara <jack@suse.cz>