net: remove synchronize_net() from netdev_set_master()

In the old days, we used to access dev->master in __netif_receive_skb()
in a rcu_read_lock section.

So one synchronize_net() call was needed in netdev_set_master() to make
sure another cpu could not use old master while/after we release it.

We now use netdev_rx_handler infrastructure and added one
synchronize_net() call in bond_release()/bond_release_all()

Remove the obsolete synchronize_net() from netdev_set_master() and add
one in bridge del_nbp() after its netdev_rx_handler_unregister() call.

This makes enslave -d a bit faster.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
CC: Jiri Pirko <jpirko@redhat.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_if.c

@@ -147,6 +147,7 @@ static void del_nbp(struct net_bridge_port *p)
 	dev->priv_flags &= ~IFF_BRIDGE_PORT;
 
 	netdev_rx_handler_unregister(dev);
+	synchronize_net();
 
 	netdev_set_master(dev, NULL);
 

net/core/dev.c

@@ -4294,10 +4294,8 @@ int netdev_set_master(struct net_device *slave, struct net_device *master)
 
 	slave->master = master;
 
-	if (old) {
-		synchronize_net();
+	if (old)
 		dev_put(old);
-	}
 	return 0;
 }
 EXPORT_SYMBOL(netdev_set_master);