Fix inaccurate disk usage reporting for ext* filesystems due to reserved blocks

[ernolf]

Key Changes:

• Adjust the calculation of the used disk space to include all blocks that cannot be used, including reserved blocks.
• Modify the disk usage percentage calculation to accurately reflect the true usage relative to the total disk size.
• Update the percentage display to show up to two decimal places, providing more precise information compared to the previous integer-only display.
• Ensure that the displayed disk size corresponds to the actual size of the disk, rather than the computed sum of Used and Available blocks.

Testing:

• Adjust unit tests to verify the correct calculation of disk usage and availability.
• Manually test on various configurations to ensure accurate reporting across different scenarios.

Related Issue:

• Closes Issue Inaccurate Disk Usage Reporting for ext* Filesystems Due to Reserved Blocks #673

before:

after:

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[kesselb]

/backport to stable30

[kesselb]

/backport to stable29

[kesselb]

/backport to stable28