ipv6: tcp: fix flowlabels reflection for RST packets

In 323a53c4 ("ipv6: tcp: enable flowlabel reflection in some RST packets")
and 50a8accf ("ipv6: tcp: send consistent flowlabel in TIME_WAIT state")
we took care of IPv6 flowlabel reflections for two cases.

This patch takes care of the remaining case, when the RST packet
is sent on behalf of a 'full' socket.

In Marek use case, this was a socket in TCP_CLOSE state.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Marek Majkowski <marek@cloudflare.com>
Tested-by: Marek Majkowski <marek@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -984,8 +984,13 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 
 	if (sk) {
 		oif = sk->sk_bound_dev_if;
-		if (sk_fullsock(sk))
+		if (sk_fullsock(sk)) {
+			const struct ipv6_pinfo *np = tcp_inet6_sk(sk);
+
 			trace_tcp_send_reset(sk, skb);
+			if (np->repflow)
+				label = ip6_flowlabel(ipv6h);
+		}
 		if (sk->sk_state == TCP_TIME_WAIT)
 			label = cpu_to_be32(inet_twsk(sk)->tw_flowlabel);
 	} else {