Fix wrong dedup_table_size for legacy dedup

[tuxoko]

Motivation and Context

If we call ddt_log_load() for legacy ddt, we will end up going into ddt_log_update_stats() and filling uninitialized value into ddo_dspace. This value will then get added to dedup_table_size during ddt_get_dedup_object_stats().

Fixes #17019

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.

[behlendorf]

It looks like this is possible because ddt_log_load_one() returns 0 instead of ENOENT when the zap_lookup() for the ddt name fails. Then ddt_log_load() carries on and calls ddt_log_update_stats(). The proposed fix certainly does the job to resolve this, but it seems like allowing ENOENT to be returned would as well. @robn what do you think.

[amotin]

I would address it by making ddt_log_update_stats() to not use doi if dmu_object_info() return error.

[tuxoko]

Yeah, I added check for dmu_object_info.