Fix wrong dedup_table_size for legacy dedup

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().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17019
Closes #17699

Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Co-authored-by: Chunwei Chen <david.chen@nutanix.com>