Add ramp_time option

Sometimes it's useful to let a job settle for a little while
before taking any measurements on latency and throughput, since
the initial rate on eg a write workload may be much higher than
the longer sustained rate.

So add a ramp_time option that allows the user to specify a lead
ramp time that must have passed before fio takes any performance
numbers into account.

Suggested by "Jenkins, Lee" <Lee.Jenkins@hp.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/options.c b/options.c
index 0608e7b..8723adf 100644
--- a/options.c
+++ b/options.c
@@ -832,6 +832,12 @@
 		.help	= "Keep running until runtime/timeout is met",
 	},
 	{
+		.name	= "ramp_time",
+		.type	= FIO_OPT_STR_VAL_TIME,
+		.off1	= td_var_offset(ramp_time),
+		.help	= "Ramp up time before measuring performance",
+	},
+	{
 		.name	= "mem",
 		.alias	= "iomem",
 		.type	= FIO_OPT_STR,