Fix ddtprune causing space leak

[tuxoko]

In zio_ddt_free, if a pruned dde is still in ddt, it would do nothing and cause space leak.

Fixes #17982

Motivation and Context

Description

How Has This Been Tested?

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.

[robn]

Good catch, very subtle. Took me a while to remember it and understand what was happening, so I've suggested a bit more commentary to explain it. Thank you!

[robn]

Suggested change

	/* dde was pruned */
	/*
	* If the entry was found but the phys was not, then
	* this block must have been pruned from the dedup
	* table, and the entry refers to a later version of
	* this data. Therefore, the caller is trying to delete
	* the only stored instance of this block, and so we
	* need to do a normal (not dedup) free. Clear dde so
	* we fall into the block below.
	*/