nvme: queue ns scanning and async request from nvme_wq

To suppress the warning triggered by nvme_uninit_ctrl:
kernel: [   50.350439] nvme nvme0: rescanning
kernel: [   50.363351] ------------[ cut here]------------
kernel: [   50.363396] WARNING: CPU: 1 PID: 37 at kernel/workqueue.c:2423 check_flush_dependency+0x11f/0x130
kernel: [   50.363409] workqueue: WQ_MEM_RECLAIM
nvme-wq:nvme_del_ctrl_work [nvme_core] is flushing !WQ_MEM_RECLAIM events:nvme_scan_work [nvme_core]

This was triggered with nvme-loop, but can happen with rdma/pci as well afaict.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>