ipv6: Process unicast packet with Router Alert by checking flag in skb.

Router Alert option is marked in skb.
Previously, IP6CB(skb)->ra was set to positive value for such packets.
Since commit dd3332bf ("ipv6: Store Router Alert option in IP6CB
directly."), IP6SKB_ROUTERALERT is set in IP6CB(skb)->flags, and
the value of Router Alert option (in network byte order) is set
to IP6CB(skb)->ra for such packets.

Multicast forwarding path uses that flag and value, but unicast
forwarding path does not use the flag and misuses IP6CB(skb)->ra
value.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -381,9 +381,8 @@ int ip6_forward(struct sk_buff *skb)
 	 *	cannot be fragmented, because there is no warranty
 	 *	that different fragments will go along one path. --ANK
 	 */
-	if (opt->ra) {
-		u8 *ptr = skb_network_header(skb) + opt->ra;
-		if (ip6_call_ra_chain(skb, (ptr[2]<<8) + ptr[3]))
+	if (unlikely(opt->flags & IP6SKB_ROUTERALERT)) {
+		if (ip6_call_ra_chain(skb, ntohs(opt->ra)))
 			return 0;
 	}