[NET]: Fix race condition about network device name allocation.

Kenji Kaneshige found this race between device removal and
registration. On unregister it is possible for the old device to
exist, because sysfs file is still open. A new device with 'eth%d'
will select the same name, but sysfs kobject register will fial.

The following changes the shutdown order slightly. It hold a removes
the sysfs entries earlier (on unregister_netdevice), but holds a
kobject reference. Then when todo runs the actual last put free
happens.

Signed-off-by: Stephen Hemminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/dev.c

···
 			continue;
 		}
 
-		netdev_unregister_sysfs(dev);
 		dev->reg_state = NETREG_UNREGISTERED;
 
 		netdev_wait_allrefs(dev);
···
 		BUG_TRAP(!dev->ip6_ptr);
 		BUG_TRAP(!dev->dn_ptr);
 
-		/* It must be the very last action,
-		 * after this 'dev' may point to freed up memory.
-		 */
 		if (dev->destructor)
 			dev->destructor(dev);
+
+		/* Free network device */
+		kobject_put(&dev->dev.kobj);
 	}
 
 out:
···
 
 	/* Notifier chain MUST detach us from master device. */
 	BUG_TRAP(!dev->master);
+
+	/* Remove entries from sysfs */
+	netdev_unregister_sysfs(dev);
 
 	/* Finish processing unregister after unlock */
 	net_set_todo(dev);

net/core/net-sysfs.c

···
 #endif
 };
 
+/* Delete sysfs entries but hold kobject reference until after all
+ * netdev references are gone.
+ */
 void netdev_unregister_sysfs(struct net_device * net)
 {
-	device_del(&(net->dev));
+	struct device *dev = &(net->dev);
+
+	kobject_get(&dev->kobj);
+	device_del(dev);
 }
 
 /* Create sysfs entries for network device. */