KASAN: null-ptr-deref Read in reclaim_high

[syzbot]

syzbot has bisected this bug to:

commit 29a4b8e275d1f10c51c7891362877ef6cffae9e7
Author: Shakeel Butt <shakeelb@xxxxxxxxxx>
Date:   Wed Jan 9 22:02:21 2019 +0000

    memcg: schedule high reclaim for remote memcgs on high_work

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=155bf5db200000
start commit:   29a4b8e2 memcg: schedule high reclaim for remote memcgs on..
git tree:       linux-next
final crash:    https://syzkaller.appspot.com/x/report.txt?x=175bf5db200000
console output: https://syzkaller.appspot.com/x/log.txt?x=135bf5db200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=611f89e5b6868db
dashboard link: https://syzkaller.appspot.com/bug?extid=fa11f9da42b46cea3b4a
userspace arch: amd64
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14259017400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=141630a0c00000

Reported-by: syzbot+fa11f9da42b46cea3b4a@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 29a4b8e2 ("memcg: schedule high reclaim for remote memcgs on  
high_work")