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/ipv6/ip6mr.c

@@ -666,8 +666,7 @@ static int pim6_rcv(struct sk_buff *skb)
 
 	skb_tunnel_rx(skb, reg_dev);
 
-	if (netif_rx(skb) == NET_RX_DROP)
-		reg_dev->stats.rx_dropped++;
+	netif_rx(skb);
 
 	dev_put(reg_dev);
 	return 0;