ovl: expand warning in ovl_d_real()

stable inclusion
from linux-4.19.177
commit 1d775f15e900453c78eb43f5b9ff0bc10f229119

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

commit cef4cbff upstream.

There was a syzbot report with this warning but insufficient information...

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>