UPSTREAM: dm verity: factor out verity_for_bv_block()

(cherry-picked from bb4d73ac5e4f0a6c4853f35824f6cb2d396a2f9c)

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

Bug: 21893453
Change-Id: Ia7782db8077bcc46fd000f5be1919942c28b03db
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>