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>

drivers/md/md.c

@@ -6271,7 +6271,8 @@ static void __md_stop(struct mddev *mddev)
 	spin_lock(&mddev->lock);
 	mddev->pers = NULL;
 	spin_unlock(&mddev->lock);
-	pers->free(mddev, mddev->private);
+	if (mddev->private)
+		pers->free(mddev, mddev->private);
 	mddev->private = NULL;
 	if (pers->sync_request && mddev->to_remove == NULL)
 		mddev->to_remove = &md_redundancy_group;