code: public int getFreeClusterCount() {
          int result = 0;
  
          for (int i=FIRST_CLUSTER; i < lastClusterIndex; i++) {
              if (isFreeCluster(i)) result++;
          }
  
          return result;
      }
docs: Returns the number of clusters that are currently not in use by this FAT.
  This estimate does only account for clusters that are really available in
  the data portion of the file system, not for clusters that might only
  theoretically be stored in the {@code Fat}.
  
  @return the free cluster count
  @see FsInfoSector#setFreeClusterCount(long)
  @see FsInfoSector#getFreeClusterCount()
  @see BootSector#getDataClusterCount()