Use only space_left, not sqes_left

src/util/ringloop.h

@@ -68,11 +68,7 @@ public:
         struct io_uring_cqe *cqe;
         return io_uring_wait_cqe(&ring, &cqe);
     }
-    int sqes_left();
-    inline unsigned space_left()
-    {
-        return free_ring_data_ptr;
-    }
+    unsigned space_left();
     inline bool has_work()
     {
         return loop_again;