dm-verity-fec: compute target region directly

dm-verity-fec: compute target region directly

Instead of determining the target block's region by checking which block
of the k blocks being iterated over in fec_read_bufs() is equal to the
target block, instead just directly use the quotient of the division of
target_block by region_blocks.

This is the same value, just derived in a more straightforward way.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>