fix utime_since overflow

utime_since was using a long for its return which overflows when the difference
between times is more than about 35 minutes.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.c b/fio.c
index e71537f..e9b27d9 100644
--- a/fio.c
+++ b/fio.c
@@ -787,9 +787,8 @@
  */
 static void *thread_main(void *data)
 {
-	unsigned long long runtime[2];
+	unsigned long long runtime[2], elapsed;
 	struct thread_data *td = data;
-	unsigned long elapsed;
 	int clear_state;
 
 	if (!td->o.use_thread)