KASAN: use-after-free Write in afs_manage_cell

[syzbot]

syzbot suspects this issue was fixed by commit:

commit 1d0e850a49a5b56f8f3cb51e74a11e2fedb96be6
Author: David Howells <dhowells at redhat.com>
Date:   Fri Oct 16 12:21:14 2020 +0000

    afs: Fix cell removal

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15b78dba500000
start commit:   da690031 Merge branch 'i2c/for-current' of git://git.kerne..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=de7f697da23057c7
dashboard link: https://syzkaller.appspot.com/bug?extid=f59c67285cb61166a0cf
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10960a8b900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17e938cf900000

If the result looks correct, please mark the issue as fixed by replying with:

#syz fix: afs: Fix cell removal

For information about bisection process see: https://goo.gl/tpsmEJ#bisection