kernfs: fix potential null pointer dereference

mainline inclusion
from mainline-5.4-rc1
commit 2fd60da4
category: bugfix
bugzilla: 21637
CVE: NA

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

Get root safely after kn is ensureed to be not null.

Signed-off-by: Peng Wang <rocking@whu.edu.cn>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20190708151611.13242-1-rocking@whu.edu.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: yu kuai <yukuai3@huawei.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>