NVMe: embedded iod mask cleanup

Remove unused mask in nvme_alloc_iod

Signed-off-by: Chong Yuan <chong.yuan@memblaze.com>
Reviewed-by: Wenbo Wang <wenbo.wang@memblaze.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>