zfs_vnops_windows: fix volume disk extent length

[lideming]

Targeted at zfs-Windows-2.4.1-release because it is the current zfswin-2.4.1rc11 release branch. Please let me know if development is preferred.

Motivation and Context

(Task Manager showing the incorrect volume size before the fix)

The volume disk extent handler should report the same byte count used by the other disk and partition size handlers.

Description

Use refdbytes + availbytes for IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS.

Also zero-initialize several disk geometry and partition output buffers before filling the fields OpenZFS returns.

How Has This Been Tested?

By Agent:

• Tested on Windows Server 2022 with a test-signed x64 driver based on zfswin-2.4.1rc11.
• Created a file-backed pool and encrypted dataset.
• Wrote data, ran zpool sync, and verified SHA256 readback.
• zpool status remained ONLINE with no known data errors.

By Human:

• A test-signed rc10 build containing this change has been used on my daily-use Windows PC since 2026-05-13 with no observed issues. The corrected volume size path was also confirmed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[lundman]

This looks great thanks. You used the correct branch, I go back and populate the Development branch periodically but haven't had the time lately.