Import fails on NixOS with kernel 4.16+

[cawilliamson]

We're really struggling on this one over on the NixOS GitHub issue:

NixOS/nixpkgs#39225

os details:

NixOS details
system: "x86_64-linux"
host os: Linux 4.15.16, NixOS, 18.09pre135256.6c064e6b1f3 (Jellyfish)
multi-user?: yes
sandbox: no
version: nix-env (Nix) 2.0
channels(root): "nixos"
nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs
nix-channel: https://nixos.org/channels/nixos-unstable

initrd:

[  474.075882] spl: loading out-of-tree module taints kernel.
[  474.077352] SPL: Loaded module v0.7.8-1
[  474.078194] stage-1-init: loading module zfs...
[  474.081054] znvpair: module license 'CDDL' taints kernel.
[  474.081055] Disabling lock debugging due to kernel taint
[  474.696450] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[  474.697005] IP: _raw_spin_lock+0xc/0x20
[  474.697005] PGD 8000000115fee067 P4D 8000000115fee067 PUD 115fe7067 PMD 0 
[  474.697005] Oops: 0002 [#1] SMP PTI
[  474.697005] Modules linked in: zfs(PO+) zunicode(PO) zavl(PO) icp(PO) zcommon(PO) znvpair(PO) spl(O)
[  474.697005] CPU: 1 PID: 155 Comm: modprobe Tainted: P           O     4.16.1 #1-NixOS
[  474.697005] Hardware name: Dell Inc. OptiPlex 780                 /0200DY, BIOS A15 08/06/2013
[  474.697005] RIP: 0010:_raw_spin_lock+0xc/0x20
[  474.697005] RSP: 0018:ffffa2cd008bfc68 EFLAGS: 00010246
[  474.697005] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[  474.697005] RDX: 0000000000000001 RSI: 0000000000000011 RDI: 0000000000000004
[  474.697005] RBP: ffffa2cd008bfcc0 R08: 0000000000025620 R09: ffffffff85e17d93
[  474.697005] R10: 0000000000000002 R11: 8080808080808080 R12: 0000000000000004
[  474.697005] R13: 00007ffffffff000 R14: 0000000000000000 R15: ffffa2cd008bfe98
[  474.697005] FS:  00007f1e11700740(0000) GS:ffff9f30dbc40000(0000) knlGS:0000000000000000
[  474.697005] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  474.697005] CR2: 0000000000000004 CR3: 0000000115f62000 CR4: 00000000000406e0
[  474.697005] Call Trace:
[  474.697005]  vn_set_pwd+0xda/0x130 [spl]
[  474.697005]  ? 0xffffffffc055a000
[  474.697005]  _init+0x12/0x2000 [zfs]
[  474.697005]  do_one_initcall+0x4e/0x18d
[  474.697005]  ? _cond_resched+0x15/0x40
[  474.697005]  ? kmem_cache_alloc_trace+0x15a/0x1b0
[  474.697005]  ? do_init_module+0x22/0x201
[  474.697005]  do_init_module+0x5b/0x201
[  474.697005]  load_module+0x2330/0x2910
[  474.697005]  ? SYSC_init_module+0x134/0x160
[  474.697005]  ? _cond_resched+0x15/0x40
[  474.697005]  SYSC_init_module+0x134/0x160
[  474.697005]  do_syscall_64+0x67/0x120
[  474.697005]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[  474.697005] RIP: 0033:0x7f1e10ddbdba
[  474.697005] RSP: 002b:00007ffcb87841c8 EFLAGS: 00000246 ORIG_RAX: 00000000000000af
[  474.697005] RAX: ffffffffffffffda RBX: 0000000000a69860 RCX: 00007f1e10ddbdba
[  474.697005] RDX: 000000000041ac66 RSI: 00000000003e15a8 RDI: 00000000013784e0
[  474.697005] RBP: 000000000041ac66 R08: 00000000017584e0 R09: 0000000000000000
[  474.697005] R10: 0000000000000000 R11: 0000000000000246 R12: 00000000013784e0
[  474.697005] R13: 0000000000a713e0 R14: 0000000000040000 R15: 0000000000a69860
[  474.697005] Code: ff 01 00 00 75 05 48 89 d8 5b c3 e8 1f 3a af ff 48 89 d8 5b c3 66 2e 0f 1f 84 00 00 00 00 00 66 66 66 66 90 31 c0 ba 01 00 00 00 <f0> 0f b1 17 85 c0 75 02 f3 c3 89 c6 e8 f3 24 af ff 66 90 c3 66 
[  474.697005] RIP: _raw_spin_lock+0xc/0x20 RSP: ffffa2cd008bfc68
[  474.697005] CR2: 0000000000000004
[  474.941774] ---[ end trace 204a3dd41abdae86 ]---

If anyone can help us out with this it would be awesome! 👍