tun/tap: Add the missed return value check of register_netdevice_notifier

There is some codes of tun/tap module which did not check the return
value of register_netdevice_notifier. Add the check now.
Signed-off-by: NTonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2598,8 +2598,16 @@ static int __init tun_init(void)
 		goto err_misc;
 	}
 
-	register_netdevice_notifier(&tun_notifier_block);
+	ret = register_netdevice_notifier(&tun_notifier_block);
+	if (ret) {
+		pr_err("Can't register netdevice notifier\n");
+		goto err_notifier;
+	}
+
 	return  0;
+
+err_notifier:
+	misc_deregister(&tun_miscdev);
 err_misc:
 	rtnl_link_unregister(&tun_link_ops);
 err_linkops: