Bug #26527 LOAD DATA INFILE extremely slow with partitioned table

mysql_load calls hander::start_bulk_insert(0) to prepare caches
for big inserts. As ha_partition didn't allow these caches
for underlaying tables, the inserts were much slower

sql/ha_partition.cc

@@ -2865,12 +2865,7 @@ void ha_partition::start_bulk_insert(ha_rows rows)
   handler **file;
   DBUG_ENTER("ha_partition::start_bulk_insert");
 
-  if (!rows)
-  {
-    /* Avoid allocation big caches in all underlaying handlers */
-    DBUG_VOID_RETURN;
-  }
-  rows= rows/m_tot_parts + 1;
+  rows= rows ? rows/m_tot_parts + 1 : 0;
   file= m_file;
   do
   {