null_zone_no
first_zone = null_zone_no(dev, sector);
struct nullb_zone *zone = &dev->zones[null_zone_no(dev, sector)];
unsigned int zno = null_zone_no(dev, sector);
zone_no = null_zone_no(dev, sector);
zone = &dev->zones[null_zone_no(dev, sector)];
zone_no = null_zone_no(dev, sector);