Use io_uring SQ size for ringloop capacity - otherwise get_sqe could return NULL when space_left() was > 0 under load

Raise default io_uring size to 1024 for the same effective capacity as previously

src/nbd_proxy.cpp

@@ -225,7 +225,7 @@ public:
             cfg = obj;
         }
         // Create client
-        ringloop = new ring_loop_t(512);
+        ringloop = new ring_loop_t(RINGLOOP_DEFAULT_SIZE);
         epmgr = new epoll_manager_t(ringloop);
         cli = new cluster_client_t(ringloop, epmgr->tfd, cfg);
         if (!inode)