netdevice: add queue selection fallback handler for ndo_select_queue

Add a new argument for ndo_select_queue() callback that passes a
fallback handler. This gets invoked through netdev_pick_tx();
fallback handler is currently __netdev_pick_tx() as most drivers
invoke this function within their customized implementation in
case for skbs that don't need any special handling. This fallback
handler can then be replaced on other call-sites with different
queue selection methods (e.g. in packet sockets, pktgen etc).

This also has the nice side-effect that __netdev_pick_tx() is
then only invoked from netdev_pick_tx() and export of that
function to modules can be undone.

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/lantiq_etop.c

@@ -619,7 +619,7 @@ ltq_etop_set_multicast_list(struct net_device *dev)
 
 static u16
 ltq_etop_select_queue(struct net_device *dev, struct sk_buff *skb,
-		      void *accel_priv)
+		      void *accel_priv, select_queue_fallback_t fallback)
 {
 	/* we are currently only using the first queue */
 	return 0;