udf: Unify types in anchor block detection

When detecting last recorded block and from it derived anchor block
position, we were mixing unsigned long, u32, and sector_t types. Since
udf supports only 32-bit block numbers this is harmless but sometimes
makes things awkward. Convert everything to udf_pblk_t and also handle
the situation when block device size would not fit into udf_pblk_t.

Signed-off-by: Jan Kara <jack@suse.cz>