nvme: add NVME_SC_CANCELLED

To properly document how we are using a negative Linux error value to
communicate request cancellations inside the driver.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>