nvme: validate admin queue before unquiesce

With a misbehaving controller it's possible we'll never
enter the live state and create an admin queue. When we
fail out of reset work it's possible we failed out early
enough without setting up the admin queue. We tear down
queues after a failed reset, but needed to do some more
sanitization.

Fixes 443bd90f: "nvme: host: unquiesce queue in nvme_kill_queues()"

[  189.650995] nvme nvme1: pci function 0000:0b:00.0
[  317.680055] nvme nvme0: Device not ready; aborting reset
[  317.680183] nvme nvme0: Removing after probe failure status: -19
[  317.681258] kasan: GPF could be caused by NULL-ptr deref or user memory access
[  317.681397] general protection fault: 0000 [#1] SMP KASAN
[  317.682984] CPU: 3 PID: 477 Comm: kworker/3:2 Not tainted 4.13.0-rc1+ #5
[  317.683112] Hardware name: Gigabyte Technology Co., Ltd. Z170X-UD5/Z170X-UD5-CF, BIOS F5 03/07/2016
[  317.683284] Workqueue: events nvme_remove_dead_ctrl_work [nvme]
[  317.683398] task: ffff8803b0990000 task.stack: ffff8803c2ef0000
[  317.683516] RIP: 0010:blk_mq_unquiesce_queue+0x2b/0xa0
[  317.683614] RSP: 0018:ffff8803c2ef7d40 EFLAGS: 00010282
[  317.683716] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 1ffff1006fbdcde3
[  317.683847] RDX: 0000000000000038 RSI: 1ffff1006f5a9245 RDI: 0000000000000000
[  317.683978] RBP: ffff8803c2ef7d58 R08: 1ffff1007bcdc974 R09: 0000000000000000
[  317.684108] R10: 1ffff1007bcdc975 R11: 0000000000000000 R12: 00000000000001c0
[  317.684239] R13: ffff88037ad49228 R14: ffff88037ad492d0 R15: ffff88037ad492e0
[  317.684371] FS:  0000000000000000(0000) GS:ffff8803de6c0000(0000) knlGS:0000000000000000
[  317.684519] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  317.684627] CR2: 0000002d1860c000 CR3: 000000045b40d000 CR4: 00000000003406e0
[  317.684758] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  317.684888] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  317.685018] Call Trace:
[  317.685084]  nvme_kill_queues+0x4d/0x170 [nvme_core]
[  317.685185]  nvme_remove_dead_ctrl_work+0x3a/0x90 [nvme]
[  317.685289]  process_one_work+0x771/0x1170
[  317.685372]  worker_thread+0xde/0x11e0
[  317.685452]  ? pci_mmcfg_check_reserved+0x110/0x110
[  317.685550]  kthread+0x2d3/0x3d0
[  317.685617]  ? process_one_work+0x1170/0x1170
[  317.685704]  ? kthread_create_on_node+0xc0/0xc0
[  317.685785]  ret_from_fork+0x25/0x30
[  317.685798] Code: 0f 1f 44 00 00 55 48 b8 00 00 00 00 00 fc ff df 48 89 e5 41 54 4c 8d a7 c0 01 00 00 53 48 89 fb 4c 89 e2 48 c1 ea 03 48 83 ec 08 <80> 3c 02 00 75 50 48 8b bb c0 01 00 00 e8 33 8a f9 00 0f ba b3
[  317.685872] RIP: blk_mq_unquiesce_queue+0x2b/0xa0 RSP: ffff8803c2ef7d40
[  317.685908] ---[ end trace a3f8704150b1e8b4 ]---
Signed-off-by: Scott Bauer <scott.bauer@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/core.c

@@ -2712,7 +2712,8 @@ void nvme_kill_queues(struct nvme_ctrl *ctrl)
 	mutex_lock(&ctrl->namespaces_mutex);
 
 	/* Forcibly unquiesce queues to avoid blocking dispatch */
-	blk_mq_unquiesce_queue(ctrl->admin_q);
+	if (ctrl->admin_q)
+		blk_mq_unquiesce_queue(ctrl->admin_q);
 
 	list_for_each_entry(ns, &ctrl->namespaces, list) {
 		/*