net: ipv6: fix skb hash for some RST packets

The skb hash comes from sk->sk_txhash when using TCP, except for some
IPv6 RST packets. This is because in tcp_v6_send_reset when not in
TIME_WAIT the hash is taken from sk->sk_hash, while it should come from
sk->sk_txhash as those two hashes are not computed the same way.

Packetdrill script to test the above,

   0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
  +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
  +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

  +0 > (flowlabel 0x1) S 0:0(0) <...>

  // Wrong ack seq, trigger a rst.
  +0 < S. 0:0(0) ack 0 win 4000

  // Check the flowlabel matches prior one from SYN.
  +0 > (flowlabel 0x1) R 0:0(0) <...>

Fixes: 9258b8b1 ("ipv6: tcp: send consistent autoflowlabel in RST packets")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -1065,7 +1065,7 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 			if (np->repflow)
 				label = ip6_flowlabel(ipv6h);
 			priority = sk->sk_priority;
-			txhash = sk->sk_hash;
+			txhash = sk->sk_txhash;
 		}
 		if (sk->sk_state == TCP_TIME_WAIT) {
 			label = cpu_to_be32(inet_twsk(sk)->tw_flowlabel);