dm integrity: fix a crash if "recalculate" used without "internal_hash"

Recalculate can only be specified with internal_hash.

Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>