ovl: switch to mounter creds in readdir

mainline inclusion
from mainline-v5.8-rc1
commit 48bd024b
category: bugfix
bugzilla: NA
CVE: CVE-2020-16120

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

In preparation for more permission checking, override credentials for
directory operations on the underlying filesystems.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>