This also happens with the (almost) current mainline kernel,
from here:

http://kernel.ubuntu.com/~kernel-ppa/mainline/daily/current/

Commit for this build was:

ed8c37e158cb697df905d6b4933bc107c69e8936

Traceback (most recent call last):
  File "/usr/bin/cloud-init", line 618, in <module>
    sys.exit(main())
  File "/usr/bin/cloud-init", line 614, in main
    get_uptime=True, func=functor, args=(name, args))
  File "/usr/lib/python2.7/dist-packages/cloudinit/util.py", line 1875, in log_time
    ret = func(*args, **kwargs)
  File "/usr/bin/cloud-init", line 510, in status_wrapper
    atomic_write_json(status_path, status)
  File "/usr/bin/cloud-init", line 434, in atomic_write_json
    raise e
OSError: [Errno 2] No such file or directory: '/var/lib/cloud/data/tmpMBxCza'
[474208.150506] BUG: unable to handle kernel paging request at ffffc90003661288
[474208.150524] IP: [<ffffffffa013a0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[474208.150536] PGD 1b6423067 PUD 1b6424067 PMD 1b255b067 PTE 0
[474208.150544] Oops: 0002 [#1] SMP 
[474208.150549] 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 ipt_REJECT xt_LOG xt_limit nf_conntrack_ipv4 nf_defrag_ipv4 xt_tcpudp xt_conntrack nf_conntrack iptable_filter ip_tables x_tables isofs dm_crypt raid10 raid456 async_memcpy async_raid6_recov async_pq async_xor async_tx xor raid6_pq raid1 raid0 multipath linear
[474208.150602] CPU: 3 PID: 6 Comm: kworker/u16:0 Not tainted 3.15.0-999-generic #201404300254
[474208.150614] Workqueue: netns cleanup_net
[474208.150619] task: ffff8801b39d0000 ti: ffff8801b39c6000 task.ti: ffff8801b39c6000
[474208.150625] RIP: e030:[<ffffffffa013a0b6>]  [<ffffffffa013a0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[474208.150634] RSP: e02b:ffff8801b39c7c48  EFLAGS: 00010246
[474208.150639] RAX: 0000000000000000 RBX: ffff8801af4e5510 RCX: ffff8801b2040000
[474208.150645] RDX: ffffc90003661288 RSI: 0000000000000200 RDI: ffffffffa013d4e0
[474208.150651] RBP: ffff8801b39c7c58 R08: 00000000f72af2f7 R09: 0000000002eb94ae
[474208.150657] R10: ffff8801b2040070 R11: 0000000000000002 R12: ffff8801af4e5480
[474208.150664] R13: ffff8801b2040000 R14: 0000000000000000 R15: 0000000000000000
[474208.150674] FS:  00007fb3072be700(0000) GS:ffff8801becc0000(0000) knlGS:0000000000000000
[474208.150681] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[474208.150686] CR2: ffffc90003661288 CR3: 00000000f54bb000 CR4: 0000000000002660
[474208.150692] Stack:
[474208.150695]  ffffffff81763762 0000000000000001 ffff8801b39c7c88 ffffffffa00f2f53
[474208.150704]  0000000000000000 ffff8801b2040000 ffff88008c4e0000 ffffffffa00d1140
[474208.150712]  ffff8801b39c7ca8 ffffffffa00ea67e ffff8801b2040000 ffff88008c4e0000
[474208.150721] Call Trace:
[474208.150729]  [<ffffffff81763762>] ? _raw_spin_lock+0x12/0x50
[474208.150744]  [<ffffffffa00f2f53>] __nf_ct_ext_destroy+0x43/0x60 [nf_conntrack]
[474208.150755]  [<ffffffffa00ea67e>] nf_conntrack_free+0x2e/0x70 [nf_conntrack]
[474208.150765]  [<ffffffffa00eb46e>] destroy_conntrack+0x9e/0xf0 [nf_conntrack]
[474208.150775]  [<ffffffffa00efbd0>] ? nf_conntrack_helper_fini+0x30/0x30 [nf_conntrack]
[474208.150785]  [<ffffffff8167f367>] nf_conntrack_destroy+0x17/0x20
[474208.150794]  [<ffffffffa00eb358>] nf_ct_iterate_cleanup+0x78/0xb0 [nf_conntrack]
[474208.150804]  [<ffffffffa00efcad>] nf_ct_l3proto_pernet_unregister+0x1d/0x20 [nf_conntrack]
[474208.150814]  [<ffffffffa00cf53d>] ipv4_net_exit+0x1d/0x60 [nf_conntrack_ipv4]
[474208.150822]  [<ffffffff816450b8>] ops_exit_list.isra.1+0x38/0x60
[474208.150828]  [<ffffffff816459c2>] cleanup_net+0x112/0x230
[474208.150837]  [<ffffffff81087b4f>] process_one_work+0x17f/0x4c0
[474208.150843]  [<ffffffff81088a7b>] worker_thread+0x11b/0x3d0
[474208.150850]  [<ffffffff81088960>] ? manage_workers.isra.21+0x190/0x190
[474208.150857]  [<ffffffff8108fb49>] kthread+0xc9/0xe0
[474208.150863]  [<ffffffff8108fa80>] ? flush_kthread_worker+0xb0/0xb0
[474208.150871]  [<ffffffff8176cc7c>] ret_from_fork+0x7c/0xb0
[474208.150877]  [<ffffffff8108fa80>] ? flush_kthread_worker+0xb0/0xb0
[474208.150882] 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 d4 13 a0 e8 24 94 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 
[474208.150936] RIP  [<ffffffffa013a0b6>] nf_nat_cleanup_conntrack+0x46/0x70 [nf_nat]
[474208.150945]  RSP <ffff8801b39c7c48>
[474208.150949] CR2: ffffc90003661288
[474208.150957] ---[ end trace 7ab56606cd4d25a0 ]---
[474208.150961] Kernel panic - not syncing: Fatal exception in interrupt