slip: not call free_netdev before rtnl_unlock in slip_open

As the description before netdev_run_todo, we cannot call free_netdev
before rtnl_unlock, fix it by reorder the code.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/slip/slip.c

@@ -863,7 +863,10 @@ err_free_chan:
 	tty->disc_data = NULL;
 	clear_bit(SLF_INUSE, &sl->flags);
 	sl_free_netdev(sl->dev);
+	/* do not call free_netdev before rtnl_unlock */
+	rtnl_unlock();
 	free_netdev(sl->dev);
+	return err;
 
 err_exit:
 	rtnl_unlock();