Clear ->rate_bytes on do_io() loop

We don't do rates across loops, so remember to clear ->rate_bytes
when we start a new loop.

Problem report from "Ananiev, Leonid I" <leonid.i.ananiev@intel.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.c b/fio.c
index 36dde3a..cc65e3d 100644
--- a/fio.c
+++ b/fio.c
@@ -636,6 +636,7 @@
 	td->ts.stat_io_bytes[0] = td->ts.stat_io_bytes[1] = 0;
 	td->this_io_bytes[0] = td->this_io_bytes[1] = 0;
 	td->zone_bytes = 0;
+	td->rate_bytes = 0;
 
 	td->last_was_sync = 0;