WARNING in xfrm_policy_insert

Hello,

syzbot hit the following crash on net-next commit
6c677750f22db3bb466a95f08b91a1cca8323146 (Tue Feb 13 10:29:05 2018 +0000)
mlxsw: spectrum: Use NL_SET_ERR_MSG_MOD

So far this crash happened 3 times on net-next, upstream.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+5cfc132a76d844973259@syzkaller.appspotmail.com
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WARNING: CPU: 0 PID: 9447 at net/xfrm/xfrm_policy.c:753  
xfrm_policy_insert+0x883/0x1140 net/xfrm/xfrm_policy.c:753
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 9447 Comm: syzkaller232283 Not tainted 4.16.0-rc1+ #225
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+0x194/0x257 lib/dump_stack.c:53
  panic+0x1e4/0x41c kernel/panic.c:183
  __warn+0x1dc/0x200 kernel/panic.c:547
  report_bug+0x211/0x2d0 lib/bug.c:184
  fixup_bug.part.11+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+0x22/0x40 arch/x86/entry/entry_64.S:988
RIP: 0010:xfrm_policy_insert+0x883/0x1140 net/xfrm/xfrm_policy.c:753
RSP: 0018:ffff8801cee97250 EFLAGS: 00010293
RAX: ffff8801c6b7a380 RBX: ffff8801c6a6d240 RCX: ffffffff84bc7033
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8801c6a6d460
RBP: ffff8801cee973e0 R08: 0000000000000000 R09: 0000000000000004
R10: ffff8801cee97108 R11: 0000000000000004 R12: dffffc0000000000
R13: 0000000000000000 R14: 000000000000007f R15: 0000000000000000
  xfrm_add_policy+0x3dc/0x8a0 net/xfrm/xfrm_user.c:1605
  xfrm_user_rcv_msg+0x41c/0x860 net/xfrm/xfrm_user.c:2595
  netlink_rcv_skb+0x14b/0x380 net/netlink/af_netlink.c:2442
  xfrm_netlink_rcv+0x6f/0x90 net/xfrm/xfrm_user.c:2603
  netlink_unicast_kernel net/netlink/af_netlink.c:1308 [inline]
  netlink_unicast+0x4c4/0x6b0 net/netlink/af_netlink.c:1334
  netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1897
  sock_sendmsg_nosec net/socket.c:630 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:640
  ___sys_sendmsg+0x767/0x8b0 net/socket.c:2047
  __sys_sendmsg+0xe5/0x210 net/socket.c:2081
  SYSC_sendmsg net/socket.c:2092 [inline]
  SyS_sendmsg+0x2d/0x50 net/socket.c:2088
  do_syscall_64+0x282/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x26/0x9b
RIP: 0033:0x446cf9
RSP: 002b:00007fb756bb7db8 EFLAGS: 00000293 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000006e168c RCX: 0000000000446cf9
RDX: 0000000000000000 RSI: 000000002014dfc8 RDI: 0000000000000009
RBP: 00000000006e1688 R08: 00007fb756bb8700 R09: 0000000000000000
R10: 00007fb756bb8700 R11: 0000000000000293 R12: 0000000000000000
R13: 00007ffe45c5ddaf R14: 00007fb756bb89c0 R15: 0000000000000008
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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