[syzbot] WARNING in drm_gem_shmem_vm_open

[syzbot]

syzbot has bisected this issue to:

commit ea40d7857d5250e5400f38c69ef9e17321e9c4a2
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Fri Oct 9 23:21:56 2020 +0000

    drm/vkms: fbdev emulation support

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11c31d55300000
start commit:   614cb2751d31 Merge tag 'trace-v5.14-rc6' of git://git.kern..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=13c31d55300000
console output: https://syzkaller.appspot.com/x/log.txt?x=15c31d55300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=96f0602203250753
dashboard link: https://syzkaller.appspot.com/bug?extid=91525b2bd4b5dff71619
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=122bce0e300000

Reported-by: syzbot+91525b2bd4b5dff71619 at syzkaller.appspotmail.com
Fixes: ea40d7857d52 ("drm/vkms: fbdev emulation support")

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