nvme-multipath: fix lockdep WARN due to partition scan work

Blktests test cases nvme/014, 057 and 058 fail occasionally due to a
lockdep WARN. As reported in the Closes tag URL, the WARN indicates that
a deadlock can happen due to the dependency among disk->open_mutex,
kblockd workqueue completion and partition_scan_work completion.

To avoid the lockdep WARN and the potential deadlock, cut the dependency
by running the partition_scan_work not by kblockd workqueue but by
nvme_wq.

Reported-by: Yi Zhang <[e-mail]>
Closes: https://lore.kernel.org/linux-block/[e-mail]/
Link: https://lore.kernel.org/linux-block/oeyzci6ffshpukpfqgztsdeke5ost5hzsuz4rrsjfmvpqcevax@5nhnwbkzbrpa/
Fixes: 1f021341eef4 ("nvme-multipath: defer partition scanning")
Signed-off-by: Shin'ichiro Kawasaki <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Keith Busch <[e-mail]>

drivers/nvme/host/multipath.c

···
 			return;
 		}
 		nvme_add_ns_head_cdev(head);
-		kblockd_schedule_work(&head->partition_scan_work);
+		queue_work(nvme_wq, &head->partition_scan_work);
 	}
 
 	nvme_mpath_add_sysfs_link(ns->head);