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: b6c1c574 ("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>