NVMe: Fix obtaining command result

Replaces req->sense_len usage, which is not owned by the LLD, to
req->special to contain the command result for driver created commands,
and sets the result unconditionally on completion.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@fb.com>
Fixes: d29ec824 ("nvme: submit internal commands through the block layer")
Signed-off-by: Jens Axboe <axboe@fb.com>