Backport of Fix available space accounting for special/dedup (#18222)

[pcd1193182]

Sponsored-by: Klara, Inc.
Sponsored-by: OSNexus

Description

This is a simple backport of @amotin 's fix for issue #18190 onto 2.2. This issue can occur on production systems in these older versions, so it is probably worth backporting this relatively small and self-contained fix to alleviate the issue.

How Has This Been Tested?

Verified no regressions using the ZFS test suite and tested that it had the desired effect on systems experiencing a critical space shortage but with ample special class space.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.

[behlendorf]

The change itself looks right. It seems we'll need to backport a few CI changes as well to get a proper test run.