[NET]: Add synchronize_net.

Currently it takes then releases BR_NETPROTO_LOCK as a writer.
Later it will be converted to RCUs synchronize_kernel().

All call sites have been verified to be sleep friendly.

include/linux/netdevice.h

@@ -486,6 +486,7 @@ extern int		dev_close(struct net_device *dev);
 extern int		dev_queue_xmit(struct sk_buff *skb);
 extern int		register_netdevice(struct net_device *dev);
 extern int		unregister_netdevice(struct net_device *dev);
+extern void		synchronize_net(void);
 extern int 		register_netdevice_notifier(struct notifier_block *nb);
 extern int		unregister_netdevice_notifier(struct notifier_block *nb);
 extern int		call_netdevice_notifiers(unsigned long val, void *v);

net/core/dev.c

@@ -2646,6 +2646,13 @@ int netdev_finish_unregister(struct net_device *dev)
 	return 0;
 }
 
+/* Synchronize with packet receive processing. */
+void synchronize_net(void) 
+{
+	br_write_lock_bh(BR_NETPROTO_LOCK);
+	br_write_unlock_bh(BR_NETPROTO_LOCK);
+}
+
 /**
  *	unregister_netdevice - remove device from the kernel
  *	@dev: device
@@ -2688,10 +2695,7 @@ int unregister_netdevice(struct net_device *dev)
 		return -ENODEV;
 	}
 
-	/* Synchronize to net_rx_action. */
-	br_write_lock_bh(BR_NETPROTO_LOCK);
-	br_write_unlock_bh(BR_NETPROTO_LOCK);
-
+	synchronize_net();
 
 #ifdef CONFIG_NET_FASTROUTE
 	dev_clear_fastroute(dev);

net/netsyms.c

@@ -544,6 +544,7 @@ EXPORT_SYMBOL(call_netdevice_notifiers);
 EXPORT_SYMBOL(loopback_dev);
 EXPORT_SYMBOL(register_netdevice);
 EXPORT_SYMBOL(unregister_netdevice);
+EXPORT_SYMBOL(synchronize_net);
 EXPORT_SYMBOL(netdev_state_change);
 EXPORT_SYMBOL(dev_new_index);
 EXPORT_SYMBOL(dev_get_by_flags);