BACKPORT: 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.

Merge conflicts in:
drivers/md/dm-verity-target.c

Bug: 27175947

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: I06c5e22a7f90a136b40f91521061f445981b5872
2 files changed