[syzbot] [mm?] kernel BUG in filemap_unaccount_folio

[syzbot]

syzbot has bisected this issue to:

commit 5c074eeabbd332b11559f7fc1e89d456f94801fb
Author: Gerd Hoffmann <kraxel at redhat.com>
Date:   Wed Nov 14 12:20:29 2018 +0000

    udmabuf: set read/write flag when exporting

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12b21bbfa80000
start commit:   db906f0ca6bb Merge tag 'phy-for-6.6' of git://git.kernel.o..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=11b21bbfa80000
console output: https://syzkaller.appspot.com/x/log.txt?x=16b21bbfa80000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3bd57a1ac08277b0
dashboard link: https://syzkaller.appspot.com/bug?extid=17a207d226b8a5fb0fd9
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11609f38680000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14c1fc00680000

Reported-by: syzbot+17a207d226b8a5fb0fd9 at syzkaller.appspotmail.com
Fixes: 5c074eeabbd3 ("udmabuf: set read/write flag when exporting")

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