WARNING in dma_buf_vunmap

[syzbot]

syzbot suspects this bug was fixed by commit:

commit 62dcb4f41836bd3c44b5b651bb6df07ea4cb1551
Author: Hans Verkuil <hverkuil-cisco at xs4all.nl>
Date:   Thu Nov 8 12:23:37 2018 +0000

     media: vb2: check memory model for VIDIOC_CREATE_BUFS

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=116af11c600000
start commit:   d41217aa Merge tag 'pci-v4.20-fixes-1' of git://git.kernel..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
dashboard link: https://syzkaller.appspot.com/bug?extid=a9317fe7ad261fc76b88
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16f7b6f5400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105a2783400000

If the result looks correct, please mark the bug fixed by replying with:

#syz fix: media: vb2: check memory model for VIDIOC_CREATE_BUFS

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