async: fix re_async_cancel mqueue handling (#995)

Since we can't clear/revert a mqueue_push, free list handling has always be done by async queueh and not within re_async_cancel. Otherwise job is reused and maybe executed twice or early.

src/async/async.c

@@ -351,7 +351,8 @@ void re_async_cancel(struct re_async *async, intptr_t id)
 		w->workh = NULL;
 		w->cb	 = NULL;
 		w->arg	 = mem_deref(w->arg);
-		list_move(&w->le, &async->freel);
+		/* No move to free list since queueh must always handled if
+		 * mqueue_push is called */
 		mtx_unlock(w->mtx);
 	}