general protection fault in get_work_pool

Hello,

syzbot hit the following crash on upstream commit
0573fed92b671bfe6a302c67bb66471b18a5004d (Fri Mar 2 18:19:57 2018 +0000)
Merge tag 'for-linus-4.16a-rc4-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip

C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+880087058dbc131a2703@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.

IPVS: ftp: loaded support on port[0] = 21
IPVS: ftp: loaded support on port[0] = 21
IPVS: ftp: loaded support on port[0] = 21
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 456 Comm: kworker/u4:4 Not tainted 4.16.0-rc3+ #337
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
RIP: 0010:get_work_pool+0x147/0x1e0 kernel/workqueue.c:718
RSP: 0018:ffff8801d8ae6e88 EFLAGS: 00010006
RAX: dffffc0000000000 RBX: 0000001ffffffe00 RCX: ffffffff8147db16
RDX: 00000003ffffffc0 RSI: ffffffff86f42900 RDI: ffffffff86ec9820
RBP: ffff8801d8ae6f08 R08: 1ffff1003b15ce12 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff1003b15cdd1
R13: 1ffff1003b15cdd5 R14: ffff8801d8ae6ee8 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffcf03439bc CR3: 0000000006e22004 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 start_flush_work kernel/workqueue.c:2808 [inline]
 flush_work+0xfc/0x8f0 kernel/workqueue.c:2870
 xfrm_policy_fini+0xb1/0x560 net/xfrm/xfrm_policy.c:2916
 xfrm_net_exit+0x1d/0x70 net/xfrm/xfrm_policy.c:2977
 ops_exit_list.isra.6+0xae/0x150 net/core/net_namespace.c:142
 cleanup_net+0x6a1/0xcb0 net/core/net_namespace.c:517
 process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
 worker_thread+0x223/0x1990 kernel/workqueue.c:2247
 kthread+0x33c/0x400 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Code: 00 48 83 c4 60 5b 41 5c 41 5d 41 5e 5d c3 48 81 e3 00 fe ff ff e8 2a 19 29 00 48 89 da 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 75 7f 48 8b 1b eb a8 e8 09 19 29 00 e8 f4 53 15 00
RIP: get_work_pool+0x147/0x1e0 kernel/workqueue.c:718 RSP: ffff8801d8ae6e88
---[ end trace ac759106a81dd957 ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzkaller@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
If you want to test a patch for this bug, please reply with:
#syz test: git://repo/address.git branch
and provide the patch inline or as an attachment.
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug report.
Note: all commands must start from beginning of the line in the email body.