net: reflect mark on tcp syn ack packets

SYN-ACK responses on a server in response to a SYN from a client
did not get the injected skb mark that was tagged on the SYN packet.

Fixes: 84f39b08 ("net: support marking accepting TCP sockets")
Reviewed-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -173,7 +173,8 @@ int ip_build_and_send_pkt(struct sk_buff *skb, const struct sock *sk,
 	}
 
 	skb->priority = sk->sk_priority;
-	skb->mark = sk->sk_mark;
+	if (!skb->mark)
+		skb->mark = sk->sk_mark;
 
 	/* Send it out. */
 	return ip_local_out(net, skb->sk, skb);

net/ipv4/tcp_output.c

@@ -3213,6 +3213,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	tcp_ecn_make_synack(req, th);
 	th->source = htons(ireq->ir_num);
 	th->dest = ireq->ir_rmt_port;
+	skb->mark = ireq->ir_mark;
 	/* Setting of flags are superfluous here for callers (and ECE is
 	 * not even correctly set)
 	 */