dm verity: fix biovecs hash calculation regression

Commit 003b5c57 ("block: Convert drivers
to immutable biovecs") incorrectly converted biovec iteration in
dm-verity to always calculate the hash from a full biovec, but the
function only needs to calculate the hash from part of the biovec (up to
the calculated "todo" value).

Fix this issue by limiting hash input to only the requested data size.

This problem was identified using the cryptsetup regression test for
veritysetup (verity-compat-test).

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.14+