nvme: refactor ns->ctrl by request

Just for current code in nvme_cleanup_cmd(), we don't have to get
namespace instance, but we need controller instance.

Controller instance can be retrieved by namespace instance, but it can
be directly accessed by nvme_request instance from request.

	ctrl = nvme_req(req)->ctrl;

We don't have to go around namespace instance from request instance
through gendisk.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>