The old _do_fork() helper has been removed in favor of kernel_clone().
Here correct some comments which still contain _do_fork()

Link: https://lore.kernel.org/r/20210111104807.18022-1-yanfei.xu@windriver.com
Cc: christian@brauner.io
Cc: linux-kernel@vger.kernel.org
Acked-by: NChristian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: NYanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: NChristian Brauner <christian.brauner@ubuntu.com>