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).

Bug: 21893453
Change-Id: If5e416de81b7f8e7a7e20fb9fcc723af19b8067d
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>