nvme: use a work item to submit async event requests

nvme: use a work item to submit async event requests

Use a dedicated work item to submit async event requests instead of the
global kthread.  This simplifies the code and reduces the latencies to
resubmit a request once an even notification happened.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 9396dec916c052855dbb5b876c13d163df397319)

Orabug: 25130845

Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>