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/fio_engine.cpp

@@ -130,7 +130,7 @@ static int bs_init(struct thread_data *td)
                 config[p.first] = p.second.dump();
         }
     }
-    bsd->ringloop = new ring_loop_t(512);
+    bsd->ringloop = new ring_loop_t(RINGLOOP_DEFAULT_SIZE);
     bsd->epmgr = new epoll_manager_t(bsd->ringloop);
     bsd->bs = new blockstore_t(config, bsd->ringloop, bsd->epmgr->tfd);
     while (1)