Returns mft zone size in clusters.
        From ntfs_progs.1.22.

def _get_mft_zone_size(self, num_clusters, mft_zone_multiplier=1):
        """Returns mft zone size in clusters.
        From ntfs_progs.1.22."""

        sizes = {
            4: num_clusters >> 1,           # 50%
            3: (num_clusters * 3) >> 3,     # 37,5%
            2: num_clusters >> 2,           # 25%
        }

        return sizes.get(mft_zone_multiplier, num_clusters >> 3)