Rename ddir_nr -> ddir_seq_nr

Another mode will be added, so lets rename this one internally
to make the distinction clear.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/options.c b/options.c
index 9230767..c9fb886 100644
--- a/options.c
+++ b/options.c
@@ -203,9 +203,9 @@
 	struct thread_data *td = data;
 	char *nr = get_opt_postfix(str);
 
-	td->o.ddir_nr = 1;
+	td->o.ddir_seq_nr = 1;
 	if (nr) {
-		td->o.ddir_nr = atoi(nr);
+		td->o.ddir_seq_nr = atoi(nr);
 		free(nr);
 	}