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

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).

Change-Id: If6d2f27b290adf14fa77f3745fdc13aaa417c8dc
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 03045cbafa2d663ad8d0a583ac219d202d824344)