dm-verity: fix dm_is_verity_target() when dm-verity is builtin

When CONFIG_DM_VERITY=y, dm_is_verity_target() returned true for any
builtin dm target, not just dm-verity.  Fix this by checking for
verity_target instead of THIS_MODULE (which is NULL for builtin code).

Fixes: b6c1c57 ("dm: Add verity helpers for LoadPin")
Cc: stable@vger.kernel.org
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Author: Eric Biggers

drivers/md/dm-verity-target.c

@@ -1521,14 +1521,6 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	return r;
 }
 
-/*
- * Check whether a DM target is a verity target.
- */
-bool dm_is_verity_target(struct dm_target *ti)
-{
-	return ti->type->module == THIS_MODULE;
-}
-
 /*
  * Get the verity mode (error behavior) of a verity target.
  *
@@ -1582,6 +1574,14 @@ static struct target_type verity_target = {
 };
 module_dm(verity);
 
+/*
+ * Check whether a DM target is a verity target.
+ */
+bool dm_is_verity_target(struct dm_target *ti)
+{
+	return ti->type == &verity_target;
+}
+
 MODULE_AUTHOR("Mikulas Patocka <mpatocka@redhat.com>");
 MODULE_AUTHOR("Mandeep Baines <msb@chromium.org>");
 MODULE_AUTHOR("Will Drewry <wad@chromium.org>");