Limit pgbench COPY FREEZE to ordinary relations

Limit pgbench COPY FREEZE to ordinary relations

pgbench client-side data generation uses COPY FREEZE to load data for most
tables. COPY FREEZE isn't supported for partitioned tables and since pgbench
only supports partitioning pgbench_accounts, pgbench used a hard-coded check to
skip COPY FREEZE and use plain COPY for a partitioned pgbench_accounts.

If the user has manually partitioned one of the other pgbench tables, this
causes client-side data generation to error out with:

ERROR:  cannot perform COPY FREEZE on a partitioned table

Fix this by limiting COPY FREEZE to ordinary tables (RELKIND_RELATION).

Author: Sergey Tatarintsev 
Reviewed-by: Álvaro Herrera 
Discussion: https://postgr.es/m/flat/97f55fca-8a7b-4da8-b413-7d1c57010676%40postgrespro.ru