Chris,

I work with Steve and was able to reproduce with the lockdep debugging, the output as follows:

[18075576.538133] BUG: unable to handle kernel paging request at ffffc900038ebac8
[18075576.538153] IP: [<ffffffffa013d1a1>] nf_nat_cleanup_conntrack+0x41/0x70 [nf_nat]
[18075576.538166] PGD 1b6428067 PUD 1b6429067 PMD 152eae067 PTE 0
[18075576.538176] Oops: 0002 [#1] SMP 
[18075576.538183] Modules linked in: xt_nat veth tcp_diag inet_diag xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat bridge stp llc xt_owner isofs ipt_REJECT xt_LOG xt_limit nf_conntrack_ipv4 nf_defrag_ipv4 xt_tcpudp xt_conntrack nf_conntrack iptable_filter ip_tables x_tables dm_crypt microcode raid10 raid456 async_memcpy async_raid6_recov async_pq async_xor async_tx xor raid6_pq raid1 raid0 multipath linear
[18075576.538246] CPU: 6 PID: 6 Comm: kworker/u16:0 Not tainted 3.15.0-0-generic #1~lp1314274
[18075576.538260] Workqueue: netns cleanup_net
[18075576.538267] task: ffff8801b3a04440 ti: ffff8801b3a0a000 task.ti: ffff8801b3a0a000
[18075576.538274] RIP: e030:[<ffffffffa013d1a1>]  [<ffffffffa013d1a1>] nf_nat_cleanup_conntrack+0x41/0x70 [nf_nat]
[18075576.538285] RSP: e02b:ffff8801b3a0bc00  EFLAGS: 00010246
[18075576.538291] RAX: 0000000000000000 RBX: ffff8800eee261b0 RCX: 00000000040003fe
[18075576.538299] RDX: ffffc900038ebac8 RSI: 000000000000035c RDI: ffffffffa0141100
[18075576.538305] RBP: ffff8801b3a0bc08 R08: 0000000000000000 R09: 0000000000000000
[18075576.538312] R10: 0000000000000001 R11: 0000000000000000 R12: ffff8800eee26100
[18075576.538319] R13: ffff8801b238c000 R14: 000000000004c2b8 R15: ffff8801b238c048
[18075576.538330] FS:  00007f3bdf8d8740(0000) GS:ffff8801bed80000(0000) knlGS:0000000000000000
[18075576.538337] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[18075576.538343] CR2: ffffc900038ebac8 CR3: 000000008c774000 CR4: 0000000000002660
[18075576.538350] Stack:
[18075576.538354]  0000000000000001 ffff8801b3a0bc38 ffffffffa00ce852 ffffffffa00ce813
[18075576.538364]  ffff8801b238c000 ffff8801b20b8000 ffff8801b238c000 ffff8801b3a0bc58
[18075576.538374]  ffffffffa00c5415 ffff8801b238c000 ffff8801b20b8000 ffff8801b3a0bc80
[18075576.538384] Call Trace:
[18075576.538399]  [<ffffffffa00ce852>] __nf_ct_ext_destroy+0x72/0xa0 [nf_conntrack]
[18075576.538413]  [<ffffffffa00ce813>] ? __nf_ct_ext_destroy+0x33/0xa0 [nf_conntrack]
[18075576.538424]  [<ffffffffa00c5415>] nf_conntrack_free+0x25/0x60 [nf_conntrack]
[18075576.538435]  [<ffffffffa00c706d>] destroy_conntrack+0xdd/0x130 [nf_conntrack]
[18075576.538447]  [<ffffffffa00c6fb7>] ? destroy_conntrack+0x27/0x130 [nf_conntrack]
[18075576.538458]  [<ffffffff8167df55>] nf_conntrack_destroy+0x45/0x70
[18075576.538466]  [<ffffffff8167df15>] ? nf_conntrack_destroy+0x5/0x70
[18075576.538477]  [<ffffffffa00cb190>] ? nf_conntrack_helper_fini+0x30/0x30 [nf_conntrack]
[18075576.538488]  [<ffffffffa00c6b53>] nf_ct_iterate_cleanup+0x203/0x230 [nf_conntrack]
[18075576.538501]  [<ffffffffa00cb32d>] nf_ct_l3proto_pernet_unregister+0x1d/0x20 [nf_conntrack]
[18075576.538512]  [<ffffffffa00b62f9>] ipv4_net_exit+0x19/0x50 [nf_conntrack_ipv4]
[18075576.538522]  [<ffffffff81642539>] ops_exit_list.isra.1+0x39/0x60
[18075576.538530]  [<ffffffff81642f28>] cleanup_net+0x108/0x1e0
[18075576.538541]  [<ffffffff810838e4>] process_one_work+0x1f4/0x510
[18075576.538548]  [<ffffffff81083882>] ? process_one_work+0x192/0x510
[18075576.538556]  [<ffffffff810845f1>] worker_thread+0x121/0x3a0
[18075576.538566]  [<ffffffff810844d0>] ? manage_workers.isra.24+0x2a0/0x2a0
[18075576.538574]  [<ffffffff8108affc>] kthread+0xfc/0x120
[18075576.538581]  [<ffffffff8108af00>] ? kthread_create_on_node+0x230/0x230
[18075576.538590]  [<ffffffff8176da7c>] ret_from_fork+0x7c/0xb0
[18075576.538596]  [<ffffffff8108af00>] ? kthread_create_on_node+0x230/0x230
[18075576.538601] Code: 0f b7 58 12 66 85 db 74 45 48 01 c3 74 40 48 83 7b 10 00 74 39 48 c7 c7 00 11 14 a0 e8 29 6e 62 e1 48 8b 03 48 8b 53 08 48 85 c0 <48> 89 02 74 04 48 89 50 08 48 b8 00 02 20 00 00 00 ad de 48 c7 
[18075576.538659] RIP  [<ffffffffa013d1a1>] nf_nat_cleanup_conntrack+0x41/0x70 [nf_nat]
[18075576.538668]  RSP <ffff8801b3a0bc00>
[18075576.538672] CR2: ffffc900038ebac8
[18075576.538682] ---[ end trace 0fff0eb21b543ad6 ]---
[18075576.538687] Kernel panic - not syncing: Fatal exception in interrupt