udf: Verify partition map count

Verify that number of partition maps isn't insanely high which can lead
to large allocation in udf_sb_alloc_partition_maps(). All partition maps
have to fit in the LVD which is in a single block.

Reported-by:  <syzbot+478f2c1a6f0f447a46bb@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>