tipc: make media_ptr pointed netdevice valid

The 'media_ptr' pointer in bearer structure which points to network
device, is protected by RCU. So, before netdevice is released,
synchronize_net() should be involved to prevent no any user of
the netdevice on read side from accessing it after it is freed.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Tested-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/bearer.c

@@ -471,6 +471,7 @@ void tipc_disable_l2_media(struct tipc_bearer *b)
 	dev = (struct net_device *)rtnl_dereference(b->media_ptr);
 	RCU_INIT_POINTER(b->media_ptr, NULL);
 	RCU_INIT_POINTER(dev->tipc_ptr, NULL);
+	synchronize_net();
 	dev_put(dev);
 }