NVMe: Fix obtaining command result

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: d29ec8241c10 ("nvme: submit internal commands through the block layer")
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit a0a931d6a2c1fbc5d5966ebf0e7a043748692c22 and
added missing pieces from d29ec8241c10eacf59c23b3828a88dbae06e7e3f
backport)
Orabug: 24532912
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>