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/[e-mail]
Acked-by: Oliver Hartkopp <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>

net/can/raw.c

···
 {
 	struct sock *sk = sock->sk;
 	struct raw_sock *ro = raw_sk(sk);
+	struct sockcm_cookie sockc;
 	struct sk_buff *skb;
 	struct net_device *dev;
 	int ifindex;
···
 	if (err < 0)
 		goto free_skb;
 
-	skb_setup_tx_timestamp(skb, sk->sk_tsflags);
+	sockcm_init(&sockc, sk);
+	if (msg->msg_controllen) {
+		err = sock_cmsg_send(sk, msg, &sockc);
+		if (unlikely(err))
+			goto free_skb;
+	}
 
 	skb->dev = dev;
 	skb->priority = sk->sk_priority;
+	skb->tstamp = sockc.transmit_time;
+
+	skb_setup_tx_timestamp(skb, sockc.tsflags);
 
 	err = can_send(skb, ro->loopback);