nvme: make nvme_error_status private

Currently it's used by the lighnvm passthrough ioctl, but we'd like to make
it private in preparation of block layer specific error code.  Lighnvm already
returns the real NVMe status anyway, so I think we can just limit it to
returning -EIO for any status set.

This will need a careful audit from the lightnvm folks, though.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>