UPSTREAM: dm verity: move dm-verity.c to dm-verity-target.c

(cherry-pick from 03045cbafa2d663ad8d0a583ac219d202d824344)

Prepare for extending dm-verity with an optional object.  Follows the
naming convention used by other DM targets (e.g. dm-cache and dm-era).

Bug: 21893453
Change-Id: I5c7c5aa219859c563846ca6e21eab57ab9d2def3
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>