Fix #167: warmup maybe problem

Fix the --warmup-time option for the fileio test by replacing internal
counters in sb_fileio.c with sb_counters_t / sb_stat_t API used in other
benchmarks. As a result, checkpoints (and --warmup-time) now reset
fileio counters properly.

src/lua/internal/sysbench.sql.lua

@@ -56,7 +56,7 @@ typedef struct
   sql_value       *values;     /* Array of column values */
 } sql_row;
 
-/* Query type for statistics */
+/* Statistic counter types */
 
 typedef enum
 {
@@ -66,6 +66,8 @@ typedef enum
   SB_CNT_TRX,
   SB_CNT_ERROR,
   SB_CNT_RECONNECT,
+  SB_CNT_BYTES_READ,
+  SB_CNT_BYTES_WRITTEN,
   SB_CNT_MAX
 } sb_counter_type;