post-review fixes.

Magnus suggested use of DYNAMIC_STRING instead of futzing with c strings.
also making usage of TABLEs clearer in store_schema_partitions_record.


client/mysqldump.c:
  Use DYNAMIC_STRING in code for dumping tablespaces
sql/sql_show.cc:
  Make use of schema_table versus the table we're showing info about in
  store_schema_partitions_record clearer