inet6: prevent network storms caused by linux IPv6 routers

Linux IPv6 forwards unicast packets, which are link layer multicasts...
The hole was present since day one. I was 100% this check is there, but it is not.

The problem shows itself, f.e. when Microsoft Network Load Balancer runs on a network.
This software resolves IPv6 unicast addresses to multicast MAC addresses.
Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -401,6 +401,9 @@ int ip6_forward(struct sk_buff *skb)
 		goto drop;
 	}
 
+	if (skb->pkt_type != PACKET_HOST)
+		goto drop;
+
 	skb_forward_csum(skb);
 
 	/*