KASAN: use-after-free Read in soft_cursor

[syzbot]

syzbot has bisected this bug to:

commit 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
Author: Russell Currey <ruscur at russell.cc>
Date:   Mon Feb 8 04:08:20 2016 +0000

     powerpc/powernv: Remove support for p5ioc2

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1512d1bce00000
start commit:   b0d4beaa Merge branch 'next.autofs' of git://git.kernel.or..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=1712d1bce00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1312d1bce00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f07a23020fd7d21a
dashboard link: https://syzkaller.appspot.com/bug?extid=cf43fb300aa142fb024b
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1745a90ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1361042ae00000

Reported-by: syzbot+cf43fb300aa142fb024b at syzkaller.appspotmail.com
Fixes: 2de50e9674fc ("powerpc/powernv: Remove support for p5ioc2")

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