nvme-pci: NULL pointer dereference in nvme_dev_disable() on linux-next

[Christoph Hellwig]

Below is the minimal fix.  I'll see if I sort out the mess that is
probe/reset failure vs ->remove a bit better, though.

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index f94b05c585cbc..577bacdcfee08 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -5160,6 +5160,8 @@ EXPORT_SYMBOL_GPL(nvme_start_freeze);
 
 void nvme_stop_queues(struct nvme_ctrl *ctrl)
 {
+	if (!ctrl->tagset)
+		return;
 	if (!test_and_set_bit(NVME_CTRL_STOPPED, &ctrl->flags))
 		blk_mq_quiesce_tagset(ctrl->tagset);
 	else
@@ -5169,6 +5171,8 @@ EXPORT_SYMBOL_GPL(nvme_stop_queues);
 
 void nvme_start_queues(struct nvme_ctrl *ctrl)
 {
+	if (!ctrl->tagset)
+		return;
 	if (test_and_clear_bit(NVME_CTRL_STOPPED, &ctrl->flags))
 		blk_mq_unquiesce_tagset(ctrl->tagset);
 }