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).
Signed-off-by: NSami Tolvanen <samitolvanen@google.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/Makefile

@@ -16,6 +16,7 @@ dm-cache-mq-y   += dm-cache-policy-mq.o
 dm-cache-smq-y   += dm-cache-policy-smq.o
 dm-cache-cleaner-y += dm-cache-policy-cleaner.o
 dm-era-y	+= dm-era-target.o
+dm-verity-y	+= dm-verity-target.o
 md-mod-y	+= md.o bitmap.o
 raid456-y	+= raid5.o raid5-cache.o