WARNING in reconnect_path

[syzbot <syzbot+3d2f4fb8f8cb02694334@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on upstream commit
b5dbc28762fd3fd40ba76303be0c7f707826f982 (Sat Mar 31 04:53:57 2018 +0000)
Merge tag 'kbuild-fixes-v4.16-3' of  
git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=3d2f4fb8f8cb02694334

So far this crash happened 3 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5514244279762944
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-2760467897697295172
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3d2f4fb8f8cb02694334@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

WARNING: CPU: 1 PID: 8789 at fs/exportfs/expfs.c:96 clear_disconnected  
fs/exportfs/expfs.c:96 [inline]
WARNING: CPU: 1 PID: 8789 at fs/exportfs/expfs.c:96  
reconnect_path+0x48e/0x670 fs/exportfs/expfs.c:232
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 8789 Comm: syz-executor6 Not tainted 4.16.0-rc7+ #8
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x24d lib/dump_stack.c:53
  panic+0x1e4/0x41c kernel/panic.c:183
  __warn+0x1dc/0x200 kernel/panic.c:547
  report_bug+0x1f4/0x2b0 lib/bug.c:186
  fixup_bug.part.10+0x37/0x80 arch/x86/kernel/traps.c:178
  fixup_bug arch/x86/kernel/traps.c:247 [inline]
  do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
  invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:986
RIP: 0010:clear_disconnected fs/exportfs/expfs.c:96 [inline]
RIP: 0010:reconnect_path+0x48e/0x670 fs/exportfs/expfs.c:232
RSP: 0018:ffff8801d7a87ad0 EFLAGS: 00010246
RAX: 0000000000040000 RBX: ffff8801ac9ad5c0 RCX: ffffffff822aaa3e
RDX: 0000000000040000 RSI: ffffc900066cc000 RDI: ffff8801ac9ad600
RBP: ffff8801d7a87b28 R08: ffffffff8858c3c0 R09: 1ffff1003af50f1d
R10: ffff8801d7a87870 R11: 0000000000000000 R12: ffff8801ac9ad5c0
R13: dffffc0000000000 R14: ffff8801ac9ad5c0 R15: ffff880178769a20
  exportfs_decode_fh+0x28a/0x5b0 fs/exportfs/expfs.c:448
  do_handle_to_path fs/fhandle.c:152 [inline]
  handle_to_path fs/fhandle.c:207 [inline]
  do_handle_open+0x374/0x8a0 fs/fhandle.c:223
  SYSC_open_by_handle_at fs/fhandle.c:265 [inline]
  SyS_open_by_handle_at+0x27/0x30 fs/fhandle.c:256
  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x454e79
RSP: 002b:00007f73f54f1c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000130
RAX: ffffffffffffffda RBX: 00007f73f54f26d4 RCX: 0000000000454e79
RDX: 0000000000200000 RSI: 0000000020000040 RDI: 0000000000000015
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000016
R13: 0000000000000416 R14: 00000000006f82b0 R15: 0000000000000025
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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