net: add a core netdev->rx_dropped counter

In various situations, a device provides a packet to our stack and we
drop it before it enters protocol stack :
- softnet backlog full (accounted in /proc/net/softnet_stat)
- bad vlan tag (not accounted)
- unknown/unregistered protocol (not accounted)

We can handle a per-device counter of such dropped frames at core level,
and automatically adds it to the device provided stats (rx_dropped), so
that standard tools can be used (ifconfig, ip link, cat /proc/net/dev)

This is a generalization of commit 8990f468a (net: rx_dropped
accounting), thus reverting it.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipip.c

@@ -414,8 +414,7 @@ static int ipip_rcv(struct sk_buff *skb)
 
 		ipip_ecn_decapsulate(iph, skb);
 
-		if (netif_rx(skb) == NET_RX_DROP)
-			tunnel->dev->stats.rx_dropped++;
+		netif_rx(skb);
 
 		rcu_read_unlock();
 		return 0;