[IPV4]: ip_route_input panic fix

This fixes http://bugzilla.kernel.org/show_bug.cgi?id=6388
The bug is caused by ip_route_input dereferencing skb->nh.protocol of
the dummy skb passed dow from inet_rtm_getroute (Thanks Thomas for seeing
it). It only happens if the route requested is for a multicast IP
address.
Signed-off-by: NStephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2741,7 +2741,10 @@ int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr* nlh, void *arg)
 	/* Reserve room for dummy headers, this skb can pass
 	   through good chunk of routing engine.
 	 */
-	skb->mac.raw = skb->data;
+	skb->mac.raw = skb->nh.raw = skb->data;
+
+	/* Bugfix: need to give ip_route_input enough of an IP header to not gag. */
+	skb->nh.iph->protocol = IPPROTO_ICMP;
 	skb_reserve(skb, MAX_HEADER + sizeof(struct iphdr));
 
 	if (rta[RTA_SRC - 1])