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: 84f39b08d786 ("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>
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 2e61e2a..7eb252d 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -173,7 +173,8 @@
 	}
 
 	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);