UPSTREAM: dm verity: factor out structures and functions useful to separate object

Prepare for an optional verity object to make use of existing dm-verity
structures and functions.

Change-Id: Ib14c3834bfed222b33e068908fb5f71a53e1187b
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit ffa393807cd69656d5b6bc9d9622e205071cbab8)