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

In the Linux kernel, the following vulnerability has been resolved: 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.