can: raw: add support for SO_TXTIME/SCM_TXTIME

This patch calls into sock_cmsg_send() to parse the user supplied
control information into a struct sockcm_cookie. Then assign the
requested transmit time to the skb.

This makes it possible to use the Earliest TXTIME First (ETF) packet
scheduler with the CAN_RAW protocol. The user can send a CAN_RAW frame
with a TXTIME and the kernel (with the ETF scheduler) will take care
of sending it to the network interface.

Link: https://lore.kernel.org/all/20220502091946.1916211-3-mkl@pengutronix.de


Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>