Add ->done variable to force exit when we know we are done

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index b7d8582..ace4b16 100644
--- a/fio.h
+++ b/fio.h
@@ -472,6 +472,7 @@
 		os_random_state_t next_file_state;
 	};
 	int error;
+	int done;
 	pid_t pid;
 	char *orig_buffer;
 	size_t orig_buffer_size;