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.

Bug: 21893453
Change-Id: I68b32d2a2ba044b73074410d9c8d916f44fb638d
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>