can: isotp: Add error message if txqueuelen is too small

This patch adds an additional error message in case that txqueuelen is
set too small and advices the user to increase txqueuelen.

This is likely to happen even with small transfers if txqueuelen is at
default value 10 frames.

Link: https://lore.kernel.org/r/20210427052150.2308-4-menschel.p@posteo.de
Signed-off-by: Patrick Menschel <menschel.p@posteo.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/isotp.c

@@ -797,10 +797,12 @@ static enum hrtimer_restart isotp_tx_timer_handler(struct hrtimer *hrtimer)
 		can_skb_set_owner(skb, sk);
 
 		can_send_ret = can_send(skb, 1);
-		if (can_send_ret)
+		if (can_send_ret) {
 			pr_notice_once("can-isotp: %s: can_send_ret %pe\n",
 				       __func__, ERR_PTR(can_send_ret));
-
+			if (can_send_ret == -ENOBUFS)
+				pr_notice_once("can-isotp: tx queue is full, increasing txqueuelen may prevent this error\n");
+		}
 		if (so->tx.idx >= so->tx.len) {
 			/* we are done */
 			so->tx.state = ISOTP_IDLE;