nvme: update and rename nvme_cancel_io to nvme_cancel_request

nvme_cancel_io is a bit confusing (given the distinction of io/admin),
so rename it to nvme_cancel_request.

And update it a bit to pass in struct nvme_ctrl, so it can be used
by Fabrics driver also.

Signed-off-by: Ming Lin <ming.l@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Keith Busch <keith.bsuch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>