KASAN: use-after-free Read in fscache_alloc_cookie

[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=17960d58500000
start commit:   171d4ff7 Merge tag 'mmc-v5.9-rc4-2' of git://git.kernel.or..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=14560d58500000
console output: https://syzkaller.appspot.com/x/log.txt?x=10560d58500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5f4c828c9e3cef97
dashboard link: https://syzkaller.appspot.com/bug?extid=2d0585e5efcd43d113c2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16a33ad3900000

Reported-by: syzbot+2d0585e5efcd43d113c2 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