md: fix double free of mddev->private in autorun_array()

In driver/md/md.c, if the function autorun_array() is called,
the problem of double free may occur.

In function autorun_array(), when the function do_md_run() returns an
error, the function do_md_stop() will be called.

The function do_md_run() called function md_run(), but in function
md_run(), the pointer mddev->private may be freed.

The function do_md_stop() called the function __md_stop(), but in
function __md_stop(), the pointer mddev->private also will be freed
without judging null.

At this time, the pointer mddev->private will be double free, so it
needs to be judged null or not.

Signed-off-by: zhangyue <zhangyue1@kylinos.cn>
Signed-off-by: Song Liu <songliubraving@fb.com>