ovl: generalize the lower_fs[] array

mainline inclusion
from mainline-5.6-rc1
commit 07f1e596
category: bugfix
bugzilla: 37632
CVE: NA

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

Rename lower_fs[] array to fs[], extend its size by one and use index fsid
(instead of fsid-1) to access the fs[] array.

Initialize fs[0] with upper fs values. fsid 0 is reserved even with lower
only overlay, so fs[0] remains null in this case.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Conflicts:
	fs/overlayfs/super.c
	[ Non bugfix patch 1bd0a3ae("ovl: use pr_fmt auto generate
	  prefix") reconstructed code is not applied. ]

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>