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 741b91f1 ("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=741b91f1



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>