netdev: Add netdev->select_queue() method.

Devices or device layers can set this to control the queue selection
performed by dev_pick_tx().

This function runs under RCU protection, which allows overriding
functions to have some way of synchronizing with things like dynamic
->real_num_tx_queues adjustments.

This makes the spinlock prefetch in dev_queue_xmit() a little bit
less effective, but that's the price right now for correctness.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -724,6 +724,9 @@ struct net_device
 	void                    (*poll_controller)(struct net_device *dev);
 #endif
 
+	u16			(*select_queue)(struct net_device *dev,
+						struct sk_buff *skb);
+
 #ifdef CONFIG_NET_NS
 	/* Network namespace this network device is inside */
 	struct net		*nd_net;