icmp: correct return value of icmp_rcv()

Currently, icmp_rcv() always return zero on a packet delivery upcall.

To make its behavior more compliant with the way this API should be
used, this patch changes this to let it return NET_RX_SUCCESS when the
packet is proper handled, and NET_RX_DROP otherwise.
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/icmp.c

@@ -1047,12 +1047,12 @@ int icmp_rcv(struct sk_buff *skb)
 
 	if (success)  {
 		consume_skb(skb);
-		return 0;
+		return NET_RX_SUCCESS;
 	}
 
 drop:
 	kfree_skb(skb);
-	return 0;
+	return NET_RX_DROP;
 csum_error:
 	__ICMP_INC_STATS(net, ICMP_MIB_CSUMERRORS);
 error: