can: slcan: add software tx timestamps

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

Add a call to skb_tx_timestamp() in the slc_xmit() function so that
the module now supports TX software timestamps.

[1] commit 741b91f1b0ea ("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

CC: Dario Binacchi <[e-mail]>
Signed-off-by: Vincent Mailhol <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/slcan/slcan-core.c

···
 	slcan_encaps(sl, (struct can_frame *)skb->data); /* encaps & send */
 	spin_unlock(&sl->lock);
 
+	skb_tx_timestamp(skb);
+
 out:
 	kfree_skb(skb);
 	return NETDEV_TX_OK;