dm verity: factor out verity_for_bv_block()

verity_for_bv_block() will be re-used by optional dm-verity object.

Bug: 21893453
Change-Id: I82a3e6efdd95a488770a2fea6794befa8f5a35ce
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>