WARNING in close_fs_devices

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    716a685fdb89 Merge branch 'x86-hyperv-for-linus' of git://..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15c5f46f800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4f65cd4286737868
dashboard link: https://syzkaller.appspot.com/bug?extid=ceb2606025ec1cc3479c
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1251799f800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=177f3edf800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ceb2606025ec1cc3479c@xxxxxxxxxxxxxxxxxxxxxxxxx

WARNING: CPU: 1 PID: 4499 at fs/btrfs/volumes.c:1071  
close_fs_devices+0xbc7/0xfa0 fs/btrfs/volumes.c:1071
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4499 Comm: syz-executor921 Not tainted 4.17.0+ #84
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 panic+0x22f/0x4de kernel/panic.c:184
 __warn.cold.8+0x163/0x1b3 kernel/panic.c:536
 report_bug+0x252/0x2d0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x1fc/0x4d0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:close_fs_devices+0xbc7/0xfa0 fs/btrfs/volumes.c:1071
Code: a0 b4 fe 0f 0b e8 89 a0 b4 fe 0f 0b 48 89 f7 e8 8f 10 f1 fe e9 f6 f5  
ff ff e8 75 a0 b4 fe 0f 0b e9 fd fd ff ff e8 69 a0 b4 fe <0f> 0b e9 a8 fd  
ff ff 48 8b bd 58 fd ff ff e8 26 11 f1 fe e9 35 fd
RSP: 0018:ffff8801ad45e530 EFLAGS: 00010293
RAX: ffff8801adb96200 RBX: 0000000000000001 RCX: ffffffff82c4ae3d
RDX: 0000000000000000 RSI: ffffffff82c4b097 RDI: 0000000000000007
RBP: ffff8801ad45e830 R08: ffff8801adb96200 R09: 0000000000000006
R10: ffff8801adb96200 R11: 0000000000000000 R12: ffffed0035a8bcc1
R13: 1ffff10035a8bcf9 R14: ffff8801ad45e7c8 R15: ffff8801b4431688
 btrfs_close_devices+0x29/0x150 fs/btrfs/volumes.c:1085
 open_ctree+0x589/0x7898 fs/btrfs/disk-io.c:3358
 btrfs_fill_super fs/btrfs/super.c:1202 [inline]
 btrfs_mount_root+0x16df/0x1e70 fs/btrfs/super.c:1593
 mount_fs+0xae/0x328 fs/super.c:1277
 vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
 vfs_kern_mount+0x40/0x60 fs/namespace.c:1027
 btrfs_mount+0x4a1/0x213e fs/btrfs/super.c:1661
 mount_fs+0xae/0x328 fs/super.c:1277
 vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
 vfs_kern_mount fs/namespace.c:1027 [inline]
 do_new_mount fs/namespace.c:2518 [inline]
 do_mount+0x564/0x30b0 fs/namespace.c:2848
 ksys_mount+0x12d/0x140 fs/namespace.c:3064
 __do_sys_mount fs/namespace.c:3078 [inline]
 __se_sys_mount fs/namespace.c:3075 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4431da
Code: b8 08 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 fd e5 fb ff c3 66 2e 0f  
1f 84 00 00 00 00 00 66 90 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 da e5 fb ff c3 66 0f 1f 84 00 00 00 00 00
RSP: 002b:00007ffe426bfd28 EFLAGS: 00000286 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004431da
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007ffe426bfd40
RBP: 0000000000000004 R08: 00000000200006c0 R09: 000000000000000a
R10: 0000000000000001 R11: 0000000000000286 R12: 0000000000000010
R13: 0000000008100000 R14: 0031656c69662f2e R15: fe03f80fe03f80ff
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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