Add time_based option

This allows fio to keep running, even if the workload has completed.
It will simply restart the workload over and over, for as long as the
runtime setting allows.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/init.c b/init.c
index 09bbce4..b5f9abc 100644
--- a/init.c
+++ b/init.c
@@ -274,6 +274,11 @@
 		return 1;
 	}
 
+	if (!o->timeout && o->time_based) {
+		log_err("fio: time_based requires a runtime/timeout setting\n");
+		o->time_based = 0;
+	}
+
 	return 0;
 }