net: ipv6: fix TCP early demux

IPv6 needs a cookie in dst_check() call.

We need to add rx_dst_cookie and provide a family independent
sk_rx_dst_set(sk, skb) method to properly support IPv6 TCP early demux.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 379e433..879db26 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -369,6 +369,7 @@
 	__u8			rcv_tclass;
 
 	__u32			dst_cookie;
+	__u32			rx_dst_cookie;
 
 	struct ipv6_mc_socklist	__rcu *ipv6_mc_list;
 	struct ipv6_ac_socklist	*ipv6_ac_list;