net: rx_dropped accounting

Under load, netif_rx() can drop incoming packets but administrators dont
have a chance to spot which device needs some tuning (RPS activation for
example)

This patch adds rx_dropped accounting in vlans and tunnels.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ipip.c b/net/ipv4/ipip.c
index 8de8888..babd252 100644
--- a/net/ipv4/ipip.c
+++ b/net/ipv4/ipip.c
@@ -377,7 +377,10 @@
 		skb_tunnel_rx(skb, tunnel->dev);
 
 		ipip_ecn_decapsulate(iph, skb);
-		netif_rx(skb);
+
+		if (netif_rx(skb) == NET_RX_DROP)
+			tunnel->dev->stats.rx_dropped++;
+
 		rcu_read_unlock();
 		return 0;
 	}