[IPV4]: Clear skb cb on IP input

when data arrives at IP through loopback (and possibly other devices).
So the field needs to be cleared before it confuses the route code.
This was seen when running netem over loopback, but there are probably
other device cases. Maybe this should go into stable?
Signed-off-by: NStephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_input.c

@@ -428,6 +428,9 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
 		goto drop;
 	}
 
+	/* Remove any debris in the socket control block */
+	memset(&(IPCB(skb)->opt), 0, sizeof(struct ip_options));
+
 	return NF_HOOK(PF_INET, NF_IP_PRE_ROUTING, skb, dev, NULL,
 		       ip_rcv_finish);