can: v(x)can: add software tx timestamps

TX timestamps were added to the can_put_echo_skb() function of can_dev
modules in [1]. However, vcan and vxcan do not rely on that function
and as such do not offer TX timestamping.

While it could be arguable whether TX timestamps are really needed for
virtual interfaces, we prefer to still add it so that all CAN drivers,
without exception, support the software TX timestamps.

Add a call to skb_tx_timestamp() in the vcan_tx() and vxcan_xmit()
functions so that the modules now support TX software timestamps.

[1] commit 741b91f ("can: dev: can_put_echo_skb(): add software
tx timestamps")
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=741b91f1b0ea34f00f6a7d4539b767c409291fcf

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20220727101641.198847-5-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/vcan.c

@@ -99,6 +99,8 @@ static netdev_tx_t vcan_tx(struct sk_buff *skb, struct net_device *dev)
 	/* set flag whether this packet has to be looped back */
 	loop = skb->pkt_type == PACKET_LOOPBACK;
 
+	skb_tx_timestamp(skb);
+
 	if (!echo) {
 		/* no echo handling available inside this driver */
 		if (loop) {

drivers/net/can/vxcan.c

@@ -53,6 +53,8 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
 		goto out_unlock;
 	}
 
+	skb_tx_timestamp(oskb);
+
 	skb = skb_clone(oskb, GFP_ATOMIC);
 	if (skb) {
 		consume_skb(oskb);