MDEV-10388 MariaDB 10.1.x keeps (deleted) ML* files in tmpdir after LOAD DATA completes

truncate unused IO_CACHE backing store files in binlog_cache_data
to release the disk space they were occupying

sql/log.cc

@@ -263,6 +263,9 @@ class binlog_cache_data
   {
     compute_statistics();
     truncate(0);
+    if(cache_log.file != -1)
+      my_chsize(cache_log.file, 0, 0, MYF(MY_WME));
+
     changes_to_non_trans_temp_table_flag= FALSE;
     incident= FALSE;
     before_stmt_pos= MY_OFF_T_UNDEF;