can: isotp: isotp_rcv_cf(): enable RX timeout handling in listen-only mode

As reported by Thomas Wagner:

    https://github.com/hartkopp/can-isotp/issues/34

the timeout handling for data frames is not enabled when the isotp socket is
used in listen-only mode (sockopt CAN_ISOTP_LISTEN_MODE). This mode is enabled
by the isotpsniffer application which therefore became inconsistend with the
strict rx timeout rules when running the isotp protocol in the operational
mode.

This patch fixes this inconsistency by moving the return condition for the
listen-only mode behind the timeout handling code.
Reported-by: Thomas Wagner <thwa1@web.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Fixes: e057dd3f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://github.com/hartkopp/can-isotp/issues/34
Link: https://lore.kernel.org/r/20201019120229.89326-1-socketcan@hartkopp.netSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/isotp.c

@@ -569,10 +569,6 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
 		return 0;
 	}
 
-	/* no creation of flow control frames */
-	if (so->opt.flags & CAN_ISOTP_LISTEN_MODE)
-		return 0;
-
 	/* perform blocksize handling, if enabled */
 	if (!so->rxfc.bs || ++so->rx.bs < so->rxfc.bs) {
 		/* start rx timeout watchdog */
@@ -581,6 +577,10 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
 		return 0;
 	}
 
+	/* no creation of flow control frames */
+	if (so->opt.flags & CAN_ISOTP_LISTEN_MODE)
+		return 0;
+
 	/* we reached the specified blocksize so->rxfc.bs */
 	isotp_send_fc(sk, ae, ISOTP_FC_CTS);
 	return 0;