Add gtod_reduce option

This cuts down even more on gtod() calls. While the three disable_X
options halved the gettimeofday() call count, this can reduce it to
less than 1 percent of what it otherwise would have been.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 62a76b9..8863fa7 100644
--- a/io_u.c
+++ b/io_u.c
@@ -851,7 +851,8 @@
 
 out:
 	if (!td_io_prep(td, io_u)) {
-		fio_gettime(&io_u->start_time, NULL);
+		if (!td->o.disable_slat)
+			fio_gettime(&io_u->start_time, NULL);
 		return io_u;
 	}
 err_put: