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/options.c b/options.c
index 754eb81..7a81880 100644
--- a/options.c
+++ b/options.c
@@ -478,6 +478,12 @@
 		.def	= "0",
 	},
 	{
+		.name	= "time_based",
+		.type	= FIO_OPT_STR_SET,
+		.off1	= td_var_offset(time_based),
+		.help	= "Keep running until runtime/timeout is met",
+	},
+	{
 		.name	= "mem",
 		.alias	= "iomem",
 		.type	= FIO_OPT_STR,