tcp: fix RFS vs lockless listeners

Before recent TCP listener patches, we were updating listener
sk->sk_rxhash before the cloning of master socket.

children sk_rxhash was therefore correct after the normal 3WHS.

But with lockless listener, we no longer dirty/change listener sk_rxhash
as it would be racy.

We need to correctly update the child sk_rxhash, otherwise first data
packet wont hit correct cpu if RFS is used.

Fixes: 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Willem de Bruijn <willemb@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/syncookies.c

@@ -225,6 +225,7 @@ struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
 	if (child) {
 		atomic_set(&req->rsk_refcnt, 1);
+		sock_rps_save_rxhash(child, skb);
 		inet_csk_reqsk_queue_add(sk, req, child);
 	} else {
 		reqsk_free(req);

net/ipv4/tcp_minisocks.c

@@ -768,6 +768,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	if (!child)
 		goto listen_overflow;
 
+	sock_rps_save_rxhash(child, skb);
 	tcp_synack_rtt_meas(child, req);
 	inet_csk_reqsk_queue_drop(sk, req);
 	inet_csk_reqsk_queue_add(sk, req, child);