WARNING: proc registration bug in afs_manage_cell

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+b994ecf2b023f14832c1 at syzkaller.appspotmail.com

Tested on:

commit:         7530d3eb afs: Don't assert on unpurgeable server records
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git
kernel config:  https://syzkaller.appspot.com/x/.config?x=302928762dfb5528
dashboard link: https://syzkaller.appspot.com/bug?extid=b994ecf2b023f14832c1
compiler:       gcc (GCC) 10.1.0-syz 20200507

Note: testing is done by a robot and is best-effort only.