can: dev: can_put_echo_skb(): print number of echo_skb that is occupied

This patch prints the number of the occupied echo_skb, to ease
implementing and debugging of new drivers.

Link: https://lore.kernel.org/r/20200915223527.1417033-11-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev.c

@@ -463,7 +463,7 @@ void can_put_echo_skb(struct sk_buff *skb, struct net_device *dev,
 		priv->echo_skb[idx] = skb;
 	} else {
 		/* locking problem with netif_stop_queue() ?? */
-		netdev_err(dev, "%s: BUG! echo_skb is occupied!\n", __func__);
+		netdev_err(dev, "%s: BUG! echo_skb %d is occupied!\n", __func__, idx);
 		kfree_skb(skb);
 	}
 }