Improve depth marking

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index 77c45f7..979ac28 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -226,10 +226,11 @@
 	if (io_u->ddir != DDIR_SYNC)
 		td->io_issues[io_u->ddir]++;
 
-	io_u_mark_depth(td, io_u);
-
 	ret = td->io_ops->queue(td, io_u);
 
+	if (ret != FIO_Q_BUSY)
+		io_u_mark_depth(td, io_u);
+
 	if (ret == FIO_Q_QUEUED) {
 		int r;