nvme: use an integer value to Linux errno values

Use a separate integer variable to hold the signed Linux errno
values we pass back to the block layer.  Note that for pass through
commands those might still be NVMe values, but those fit into the
int as well.

Fixes: f4829a9b: ("blk-mq: fix racy updates of rq->errors")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>