ipv6: tcp: send consistent autoflowlabel in TIME_WAIT state

In case autoflowlabel is in action, skb_get_hash_flowi6()
derives a non zero skb->hash to the flowlabel.

If skb->hash is zero, a flow dissection is performed.

Since all TCP skbs sent from ESTABLISH state inherit their
skb->hash from sk->sk_txhash, we better keep a copy
of sk->sk_txhash into the TIME_WAIT socket.

After this patch, ACK or RST packets sent on behalf of
a TIME_WAIT socket have the flowlabel that was previously
used by the flow.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_timewait_sock.h

@@ -70,6 +70,7 @@ struct inet_timewait_sock {
 				tw_flowlabel	: 20,
 				tw_pad		: 2,	/* 2 bits hole */
 				tw_tos		: 8;
+	u32			tw_txhash;
 	struct timer_list	tw_timer;
 	struct inet_bind_bucket	*tw_tb;
 };