fix(pagebench): avoid CopyFail error in success case (#6443)

PR #6392 fixed CopyFail in the case where we get cancelled.
But, we also want to use `client.shutdown()` if we don't get cancelled.

pageserver/pagebench/src/cmd/getpage_latest_lsn.rs

@@ -423,8 +423,8 @@ async fn client(
     tokio::select! {
         res = do_requests => { res },
         _ = cancel.cancelled() => {
-            client.shutdown().await;
-            return;
+            // fallthrough to shutdown
         }
     }
+    client.shutdown().await;
 }