netfilter: ipv6: add IPSKB_REROUTED exclusion to NF_HOOK/POSTROUTING invocation

Similar to how IPv4's ip_output.c works, have ip6_output also check
the IPSKB_REROUTED flag. It will be set from xt_TEE for cloned packets
since Xtables can currently only deal with a single packet in flight
at a time.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Acked-by: David S. Miller <davem@davemloft.net>
[Patrick: changed to use an IP6SKB value instead of IPSKB]
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/ipv6.h

@@ -250,6 +250,7 @@ struct inet6_skb_parm {
 
 #define IP6SKB_XFRM_TRANSFORMED	1
 #define IP6SKB_FORWARDED	2
+#define IP6SKB_REROUTED		4
 };
 
 #define IP6CB(skb)	((struct inet6_skb_parm*)((skb)->cb))

net/ipv6/ip6_output.c

@@ -172,8 +172,9 @@ int ip6_output(struct sk_buff *skb)
 		return 0;
 	}
 
-	return NF_HOOK(NFPROTO_IPV6, NF_INET_POST_ROUTING, skb, NULL, dev,
-		       ip6_finish_output);
+	return NF_HOOK_COND(NFPROTO_IPV6, NF_INET_POST_ROUTING, skb, NULL, dev,
+			    ip6_finish_output,
+			    !(IP6CB(skb)->flags & IP6SKB_REROUTED));
 }
 
 /*