nvme: fix scsi translation error handling

Erorr handling for the scsi translation was completely broken, as there
were two different positive error number spaces overlapping.  Fix this
up by removing one of them, and centralizing the generation of the other
positive values in a single place.  Also fix up a few places that didn't
handle the NVMe error codes properly.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@fb.com>