[NET]: Inline net_device_stats

Network drivers which keep stats allocate their own stats structure
then write a get_stats() function to return them.  It would be nice if
this were done by default.

1) Add a new "stats" field to "struct net_device".
2) Add a new feature field to say "this driver uses the internal one"
3) Have a default "get_stats" which returns NULL if that feature not set.
4) Change callers to check result of get_stats call for NULL, not if
   ->get_stats is set.

This should not break backwards compatibility with older drivers, yet
allow modern drivers to shed some boilerplate code.

Lightly tested: works for a modified lguest network driver.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3640,9 +3640,8 @@ static struct net_device_stats *bond_get_stats(struct net_device *bond_dev)
 	read_lock_bh(&bond->lock);
 
 	bond_for_each_slave(bond, slave, i) {
-		if (slave->dev->get_stats) {
-			sstats = slave->dev->get_stats(slave->dev);
-
+		sstats = slave->dev->get_stats(slave->dev);
+		if (sstats) {
 			stats->rx_packets += sstats->rx_packets;
 			stats->rx_bytes += sstats->rx_bytes;
 			stats->rx_errors += sstats->rx_errors;