suspicious RCU usage at net/netfilter/ipset/ip_set_core.c:LINE

Hello,

syzkaller hit the following crash on  
e0e99813fb1d25fc20dab946d7bf6156a67dab5f
git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
compiler: gcc (GCC) 7.1.1 20170620
.config is attached
Raw console output is attached.
C reproducer is attached
syzkaller reproducer is attached. See https://goo.gl/kgGztJ
for information about syzkaller reproducers


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Reported-by: syzbot+36b06f219f2439fe62e1@syzkaller.appspotmail.com
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=============================
WARNING: suspicious RCU usage
4.15.0-rc6-next-20180102+ #86 Not tainted
-----------------------------
net/netfilter/ipset/ip_set_core.c:2057 suspicious  
rcu_dereference_protected() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
3 locks held by kworker/u4:0/5:
  #0:  ((wq_completion)"%s""netns"){+.+.}, at: [<000000006581c2a2>]  
process_one_work+0x71f/0x14a0 kernel/workqueue.c:2083
  #1:  (net_cleanup_work){+.+.}, at: [<0000000032ed8df0>]  
process_one_work+0x757/0x14a0 kernel/workqueue.c:2087
  #2:  (net_mutex){+.+.}, at: [<000000004beb14d8>] cleanup_net+0x139/0x8b0  
net/core/net_namespace.c:450

stack backtrace:
CPU: 0 PID: 5 Comm: kworker/u4:0 Not tainted 4.15.0-rc6-next-20180102+ #86
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x137/0x198 lib/dump_stack.c:53
  lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4585
  ip_set_net_exit+0x2c6/0x480 net/netfilter/ipset/ip_set_core.c:2057
  ops_exit_list.isra.6+0xae/0x150 net/core/net_namespace.c:142
  cleanup_net+0x3f3/0x8b0 net/core/net_namespace.c:484
  process_one_work+0x801/0x14a0 kernel/workqueue.c:2112
  worker_thread+0xe0/0x1010 kernel/workqueue.c:2246
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:524


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