This goes right back to 4.6.x:

.6.7 crash (see below)
4.7.10 crash in xfrm6_dst_ifdown
4.8.17 crash in xfrm6_dst_ifdown
4.12.14 crash (see below)
4.13.16 reports "unregister_netdevice: waiting for eth0 to become free. Usage count = 2"
4.14.157 reports "unregister_netdevice: waiting for eth0 to become free. Usage count = 2""
4.15.18 .. 5.4 hangs on socket() call

4.6.7:
[   34.457967] BUG: scheduling while atomic: kworker/u8:0/6/0x00000200
[   34.458021] Modules linked in: esp6 xfrm6_mode_transport drbg ansi_cprng seqiv esp4 xfrm4_mode_transport xfrm_user xfrm_algo l2tp_ip6 l2tp_eth l2tp_ip l2tp_netlink veth l2tp_core ip6_udp_tunnel udp_tunnel squashfs binfmt_misc dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua ppdev kvm_intel kvm irqbypass joydev input_leds snd_hda_codec_generic serio_raw snd_hda_intel snd_hda_codec parport_pc 8250_fintek parport snd_hda_core qemu_fw_cfg snd_hwdep snd_pcm snd_timer mac_hid snd soundcore sch_fq_codel virtio_rng ip_tables x_tables autofs4 btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor hid_generic usbhid hid raid6_pq libcrc32c raid1 raid0 multipath linear crct10dif_pclmul crc32_pclmul ghash_clmulni_intel qxl ttm drm_kms_helper syscopyarea sysfillrect aesni_intel sysimgblt
[   34.458086]  fb_sys_fops aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd i2c_piix4 drm psmouse pata_acpi floppy
[   34.458100] CPU: 1 PID: 6 Comm: kworker/u8:0 Not tainted 4.6.7-040607-generic #201608160432
[   34.458103] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[   34.458131] Workqueue: netns cleanup_net
[   34.458135]  0000000000000286 000000002fa171e7 ffff88007c8e7ab8 ffffffff813f7594
[   34.458139]  ffff88007fc96b80 7fffffffffffffff ffff88007c8e7ac8 ffffffff810a8f6b
[   34.458143]  ffff88007c8e7b18 ffffffff8184905b 00ff88007c8e7ae8 ffffffff8106463e
[   34.458147] Call Trace:
[   34.458161]  [<ffffffff813f7594>] dump_stack+0x63/0x8f
[   34.458166]  [<ffffffff810a8f6b>] __schedule_bug+0x4b/0x60
[   34.458185]  [<ffffffff8184905b>] __schedule+0x5eb/0x7a0
[   34.458191]  [<ffffffff8106463e>] ? kvm_sched_clock_read+0x1e/0x30
[   34.458195]  [<ffffffff81849245>] schedule+0x35/0x80
[   34.458203]  [<ffffffff8184c402>] schedule_timeout+0x1b2/0x270
[   34.458207]  [<ffffffff81848d74>] ? __schedule+0x304/0x7a0
[   34.458212]  [<ffffffff81849ca3>] wait_for_completion+0xb3/0x140
[   34.458217]  [<ffffffff810aeed0>] ? wake_up_q+0x70/0x70
[   34.458226]  [<ffffffff810e7f68>] __wait_rcu_gp+0xc8/0xf0
[   34.458231]  [<ffffffff810e9fd8>] synchronize_sched.part.58+0x38/0x50
[   34.458235]  [<ffffffff810ec570>] ? call_rcu_bh+0x20/0x20
[   34.458239]  [<ffffffff810e7e80>] ? trace_raw_output_rcu_utilization+0x60/0x60
[   34.458244]  [<ffffffff810ec643>] synchronize_sched+0x33/0x40
[   34.458251]  [<ffffffffc0510f71>] __l2tp_session_unhash+0xd1/0xe0 [l2tp_core]
[   34.458256]  [<ffffffffc051101e>] l2tp_tunnel_closeall+0x9e/0x140 [l2tp_core]
[   34.458261]  [<ffffffffc0511219>] l2tp_tunnel_delete+0x19/0x70 [l2tp_core]
[   34.458265]  [<ffffffffc05112bb>] l2tp_exit_net+0x4b/0x80 [l2tp_core]
[   34.458269]  [<ffffffff81732188>] ops_exit_list.isra.4+0x38/0x60
[   34.458273]  [<ffffffff817331e4>] cleanup_net+0x1c4/0x2a0
[   34.458281]  [<ffffffff8109ccfc>] process_one_work+0x1fc/0x490
[   34.458285]  [<ffffffff8109cfdb>] worker_thread+0x4b/0x500
[   34.458290]  [<ffffffff8109cf90>] ? process_one_work+0x490/0x490
[   34.458293]  [<ffffffff810a37c8>] kthread+0xd8/0xf0
[   34.458298]  [<ffffffff8184d522>] ret_from_fork+0x22/0x40
[   34.458302]  [<ffffffff810a36f0>] ? kthread_create_on_node+0x1b0/0x1b0
[   34.514067] ------------[ cut here ]------------

4.12.14:
[   20.760253] ------------[ cut here ]------------
[   20.760256] kernel BUG at /home/kernel/COD/linux/net/ipv6/xfrm6_policy.c:265!
[   20.760299] invalid opcode: 0000 [#1] SMP
[   20.760320] Modules linked in: appletalk psnap llc esp6 xfrm6_mode_transport esp4 xfrm4_mode_transport xfrm_user xfrm_algo l2tp_ip6 l2tp_eth l2tp_ip l2tp_netlink veth l2tp_core ip6_udp_tunnel udp_tunnel binfmt_misc dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua joydev ppdev snd_hda_codec_generic kvm_intel kvm irqbypass snd_hda_intel snd_hda_codec snd_hda_core input_leds snd_hwdep serio_raw snd_pcm snd_timer hid_generic snd soundcore parport_pc parport mac_hid qemu_fw_cfg sch_fq_codel virtio_rng ip_tables x_tables autofs4 usbhid hid btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel aes_x86_64 crypto_simd qxl glue_helper ttm cryptd drm_kms_helper psmouse
[   20.760677]  syscopyarea sysfillrect virtio_blk sysimgblt fb_sys_fops drm floppy virtio_net i2c_piix4 pata_acpi
[   20.760731] CPU: 3 PID: 49 Comm: kworker/u8:1 Not tainted 4.12.14-041214-generic #201709200843
[   20.760772] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[   20.760814] Workqueue: netns cleanup_net
[   20.760836] task: ffff8aa4bcbbad00 task.stack: ffff9dc5804c0000
[   20.760867] RIP: 0010:xfrm6_dst_ifdown+0xa0/0xb0
[   20.760890] RSP: 0018:ffff9dc5804c3be0 EFLAGS: 00010246
[   20.760916] RAX: ffff8aa4b6e6a000 RBX: ffff8aa4bc1b3500 RCX: 0000000000000000
[   20.760950] RDX: 0000000000000001 RSI: ffff8aa4b6f39000 RDI: ffff8aa4bc1b3500
[   20.760984] RBP: ffff9dc5804c3c08 R08: 0000000000000000 R09: ffffffffb49fd7a0
[   20.761017] R10: ffff9dc5804c3c70 R11: 0000000000000000 R12: ffff8aa4b6f39000
[   20.761050] R13: ffff8aa4b6f39000 R14: ffff8aa4bc1b3500 R15: 0000000000000000
[   20.761085] FS:  0000000000000000(0000) GS:ffff8aa4bfd80000(0000) knlGS:0000000000000000
[   20.761123] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   20.761150] CR2: 00007fa5cd126718 CR3: 000000007c382000 CR4: 00000000001406e0
[   20.761189] Call Trace:
[   20.761207]  dst_ifdown+0x26/0x80
[   20.761226]  dst_dev_event+0x5c/0x170
[   20.761247]  notifier_call_chain+0x4a/0x70
[   20.761269]  raw_notifier_call_chain+0x16/0x20
[   20.761293]  call_netdevice_notifiers_info+0x35/0x60
[   20.761318]  netdev_run_todo+0xcf/0x300
[   20.761340]  rtnl_unlock+0xe/0x10
[   20.761359]  default_device_exit_batch+0x153/0x180
[   20.761385]  ? do_wait_intr_irq+0x90/0x90
[   20.761408]  ops_exit_list.isra.6+0x52/0x60
[   20.761430]  cleanup_net+0x1ca/0x2b0
[   20.761451]  process_one_work+0x1e7/0x410
[   20.761472]  worker_thread+0x4a/0x410
[   20.761492]  kthread+0x125/0x140
[   20.761511]  ? process_one_work+0x410/0x410
[   20.761532]  ? kthread_create_on_node+0x70/0x70
[   20.761556]  ret_from_fork+0x25/0x30
[   20.761575] Code: f0 00 00 00 75 05 e8 10 6f 00 00 4c 89 bb 58 01 00 00 f0 41 ff 04 24 48 8b 5b 10 48 83 7b 48 00 75 d4 f0 41 ff 0c 24 eb 8e f3 c3 <0f> 0b 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 55 b9 06 00 00 
[   20.761695] RIP: xfrm6_dst_ifdown+0xa0/0xb0 RSP: ffff9dc5804c3be0
[   20.762104] ---[ end trace b22472ed4abae541 ]---