ipv4: Elide use of rt->rt_dst in ip_forward()

No matter what kind of header mangling occurs due to IP options
processing, rt->rt_dst will always equal iph->daddr in the packet.

So we can safely use iph->daddr instead of rt->rt_dst here.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_forward.c

@@ -84,7 +84,7 @@ int ip_forward(struct sk_buff *skb)
 
 	rt = skb_rtable(skb);
 
-	if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
+	if (opt->is_strictroute && iph->daddr != rt->rt_gateway)
 		goto sr_failed;
 
 	if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) &&