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

mainline inclusion
from mainline-v5.16-rc5
commit 07641b5f
category: bugfix
bugzilla: 188015, https://gitee.com/openeuler/kernel/issues/I6OERX
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=07641b5f32f6991758b08da9b1f4173feeb64f2a



--------------------------------

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>
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>