Change the ISC_R_SUCCESS to DNS_RRL_RESULT_OK in dns_rrl()

There's value mismatch between the return type of dns_rrl() that's
dns_rrl_result_t and ISC_R_SUCCESS which belongs to isc_result_t.  This
works incidentally, because DNS_RRL_RESULT_OK == ISC_R_SUCCESS.

This would break when we change isc_result_t to be static enum in
consecutive commit.  Change the value to match the type.

lib/dns/rrl.c

@@ -1113,7 +1113,7 @@ dns_rrl(dns_view_t *view, const isc_sockaddr_t *client_addr, bool is_tcp,
 			}
 		}
 		UNLOCK(&rrl->lock);
-		return (ISC_R_SUCCESS);
+		return (DNS_RRL_RESULT_OK);
 	}
 
 	/*