The problem also occurs with the mainline v3.15-rc2-trusty build:

[17345307.967478] BUG: unable to handle kernel paging request at ffffc90003777a70
[17345307.967497] IP: [<ffffffffa013f0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[17345307.967510] PGD 1b6425067 PUD 1b6426067 PMD 1b0aed067 PTE 0
[17345307.967519] Oops: 0002 [#1] SMP 
[17345307.967525] 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
[17345307.967578] CPU: 0 PID: 6 Comm: kworker/u16:0 Not tainted 3.15.0-031500rc2-generic #201404201435
[17345307.967591] Workqueue: netns cleanup_net
[17345307.967596] task: ffff8801b39d0000 ti: ffff8801b39cc000 task.ti: ffff8801b39cc000
[17345307.967601] RIP: e030:[<ffffffffa013f0b6>]  [<ffffffffa013f0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[17345307.967611] RSP: e02b:ffff8801b39cdc48  EFLAGS: 00010246
[17345307.967617] RAX: 0000000000000000 RBX: ffff8801b1cf7b10 RCX: ffff880003110000
[17345307.967624] RDX: ffffc90003777a70 RSI: 0000000000000200 RDI: ffffffffa01434e0
[17345307.967630] RBP: ffff8801b39cdc58 R08: 0000000058690aeb R09: 00000000e834b0f3
[17345307.967636] R10: ffff880003110070 R11: 0000000000000002 R12: ffff8801b1cf7a80
[17345307.967643] R13: ffff880003110000 R14: 0000000000000000 R15: 0000000000000000
[17345307.967655] FS:  00007fd6682cc740(0000) GS:ffff8801bec00000(0000) knlGS:0000000000000000
[17345307.967662] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[17345307.967667] CR2: ffffc90003777a70 CR3: 00000000058f9000 CR4: 0000000000002660
[17345307.967673] Stack:
[17345307.967677]  ffffffff8176ce52 0000000000000001 ffff8801b39cdc88 ffffffffa00d1013
[17345307.967686]  0000000000000000 ffff880003110000 ffff8800ca7b8000 ffffffffa00e5140
[17345307.967694]  ffff8801b39cdca8 ffffffffa00c867e ffff880003110000 ffff8800ca7b8000
[17345307.967703] Call Trace:
[17345307.967712]  [<ffffffff8176ce52>] ? _raw_spin_lock+0x12/0x50
[17345307.967726]  [<ffffffffa00d1013>] __nf_ct_ext_destroy+0x43/0x60 [nf_conntrack]
[17345307.967736]  [<ffffffffa00c867e>] nf_conntrack_free+0x2e/0x70 [nf_conntrack]
[17345307.967746]  [<ffffffffa00c946e>] destroy_conntrack+0x9e/0xf0 [nf_conntrack]
[17345307.967756]  [<ffffffffa00cdc40>] ? nf_conntrack_helper_fini+0x30/0x30 [nf_conntrack]
[17345307.967766]  [<ffffffff81686617>] nf_conntrack_destroy+0x17/0x20
[17345307.967775]  [<ffffffffa00c9358>] nf_ct_iterate_cleanup+0x78/0xb0 [nf_conntrack]
[17345307.967786]  [<ffffffffa00cdd1d>] nf_ct_l3proto_pernet_unregister+0x1d/0x20 [nf_conntrack]
[17345307.967796]  [<ffffffffa00e355d>] ipv4_net_exit+0x1d/0x60 [nf_conntrack_ipv4]
[17345307.967804]  [<ffffffff8164b918>] ops_exit_list.isra.1+0x38/0x60
[17345307.967811]  [<ffffffff8164c222>] cleanup_net+0x112/0x230
[17345307.967820]  [<ffffffff81085e2f>] process_one_work+0x17f/0x4c0
[17345307.967827]  [<ffffffff81086d7b>] worker_thread+0x11b/0x3d0
[17345307.967833]  [<ffffffff81086c60>] ? manage_workers.isra.21+0x190/0x190
[17345307.967841]  [<ffffffff8108de69>] kthread+0xc9/0xe0
[17345307.967846]  [<ffffffff8108dda0>] ? flush_kthread_worker+0xb0/0xb0
[17345307.967854]  [<ffffffff8177647c>] ret_from_fork+0x7c/0xb0
[17345307.967860]  [<ffffffff8108dda0>] ? flush_kthread_worker+0xb0/0xb0
[17345307.967865] Code: b7 58 12 66 85 db 74 46 0f b7 db 48 01 c3 48 83 7b 10 00 74 39 48 c7 c7 e0 34 14 a0 e8 14 db 62 e1 48 8b 03 48 8b 53 08 48 85 c0 <48> 89 02 74 04 48 89 50 08 48 ba 00 02 20 00 00 00 ad de 48 c7 
[17345307.967917] RIP  [<ffffffffa013f0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[17345307.967925]  RSP <ffff8801b39cdc48>
[17345307.967928] CR2: ffffc90003777a70
[17345307.967933] ---[ end trace 84d4f3185a40459f ]---
[17345307.967938] Kernel panic - not syncing: Fatal exception in interrupt