nvme-pci: use blk-mq mapping for unmanaged irqs

If a device is providing a single IRQ vector, the IO queue will share
that vector with the admin queue. This is an unmanaged vector, so does
not have a valid PCI IRQ affinity. Avoid trying to extract a managed
affinity in this case and let blk-mq set up the cpu:queue mapping instead.
Otherwise we'd hit the following warning when the device is using MSI:

 WARNING: CPU: 4 PID: 7 at drivers/pci/msi.c:1272 pci_irq_get_affinity+0x66/0x80
 Modules linked in: nvme nvme_core serio_raw
 CPU: 4 PID: 7 Comm: kworker/u16:0 Tainted: G        W         5.2.0-rc1+ #494
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
 Workqueue: nvme-reset-wq nvme_reset_work [nvme]
 RIP: 0010:pci_irq_get_affinity+0x66/0x80
 Code: 0b 31 c0 c3 83 e2 10 48 c7 c0 b0 83 35 91 74 2a 48 8b 87 d8 03 00 00 48 85 c0 74 0e 48 8b 50 30 48 85 d2 74 05 39 70 14 77 05 <0f> 0b 31 c0 c3 48 63 f6 48 8d 04 76 48 8d 04 c2 f3 c3 48 8b 40 30
 RSP: 0000:ffffb5abc01d3cc8 EFLAGS: 00010246
 RAX: ffff9536786a39c0 RBX: 0000000000000000 RCX: 0000000000000080
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9536781ed000
 RBP: ffff95367346a008 R08: ffff95367d43f080 R09: ffff953678c07800
 R10: ffff953678164800 R11: 0000000000000000 R12: 0000000000000000
 R13: ffff9536781ed000 R14: 00000000ffffffff R15: ffff95367346a008
 FS:  0000000000000000(0000) GS:ffff95367d400000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007fdf814a3ff0 CR3: 000000001a20f000 CR4: 00000000000006e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  blk_mq_pci_map_queues+0x37/0xd0
  nvme_pci_map_queues+0x80/0xb0 [nvme]
  blk_mq_alloc_tag_set+0x133/0x2f0
  nvme_reset_work+0x105d/0x1590 [nvme]
  process_one_work+0x291/0x530
  worker_thread+0x218/0x3d0
  ? process_one_work+0x530/0x530
  kthread+0x111/0x130
  ? kthread_park+0x90/0x90
  ret_from_fork+0x1f/0x30
 ---[ end trace 74587339d93c83c0 ]---

Fixes: 22b55601 ("nvme-pci: Separate IO and admin queue IRQ vectors")
Reported-by: Iván Chavero <ichavero@chavero.com.mx>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>

drivers/nvme/host/pci.c

@@ -464,7 +464,7 @@ static int nvme_pci_map_queues(struct blk_mq_tag_set *set)
 		 * affinity), so use the regular blk-mq cpu mapping
 		 */
 		map->queue_offset = qoff;
-		if (i != HCTX_TYPE_POLL)
+		if (i != HCTX_TYPE_POLL && offset)
 			blk_mq_pci_map_queues(map, to_pci_dev(dev->dev), offset);
 		else
 			blk_mq_map_queues(map);