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

(cherry-picked from ffa39380)

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

Bug: 21893453
Change-Id: I102463e88089eb047596d4acccc424e42c3abca5
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>