Minor fix to make rpl_loaddata_map pass without warnings.

mysql-test/suite/rpl/t/rpl_loaddata_map-master.opt:
  Correcting read_buffer_size to be a multiple of 4KiB since a
  warning is generated otherwise.

mysql-test/suite/rpl/t/rpl_loaddata_map-master.opt

---read_buffer_size=9K --max_allowed_packet=8K
+--read_buffer_size=12K --max_allowed_packet=8K