KASAN: use-after-free Write in afs_manage_cell

[syzbot]

syzbot has bisected this issue to:

commit c99c2171fc61476afac0dfb59fb2c447a01fb1e0
Author: David Howells <dhowells at redhat.com>
Date:   Thu Nov 1 23:07:27 2018 +0000

    afs: Use fs_context to pass parameters over automount

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10679fa9900000
start commit:   abb3438d Merge tag 'm68knommu-for-v5.9-rc3' of git://git.k..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=12679fa9900000
console output: https://syzkaller.appspot.com/x/log.txt?x=14679fa9900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=891ca5711a9f1650
dashboard link: https://syzkaller.appspot.com/bug?extid=f59c67285cb61166a0cf
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10d97d46900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14eb9fa9900000

Reported-by: syzbot+f59c67285cb61166a0cf at syzkaller.appspotmail.com
Fixes: c99c2171fc61 ("afs: Use fs_context to pass parameters over automount")

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