nvme: move nvme_cancel_request() to common code

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>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Keith Busch <keith.bsuch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>