BTRFS_FREE_SPACE_BITMAP_BITS
bitmap_range = fs_info->sectorsize * BTRFS_FREE_SPACE_BITMAP_BITS;
bitmap_range = cache->fs_info->sectorsize * BTRFS_FREE_SPACE_BITMAP_BITS;
bitmap_alignment = BTRFS_FREE_SPACE_BITMAP_BITS * PAGE_SIZE;