pageserver: fix buffered-writer on macos build (#10019)

## Problem

In #9693, we forgot to check
macos build. The [CI
run](https://github.com/neondatabase/neon/actions/runs/12164541897/job/33926455468)
on main showed that macos build failed with unused variables and dead
code.

## Summary of changes

- add `allow(dead_code)` and `allow(unused_variables)` to the relevant
code that is not used on macos.

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -6,7 +6,6 @@
 use std::collections::HashSet;
 use std::future::Future;
 use std::str::FromStr;
-use std::sync::Arc;
 use std::time::SystemTime;
 
 use anyhow::{anyhow, Context};
@@ -27,7 +26,7 @@ use crate::span::{
 use crate::tenant::remote_timeline_client::{remote_layer_path, remote_timelines_path};
 use crate::tenant::storage_layer::LayerName;
 use crate::tenant::Generation;
-use crate::virtual_file::{on_fatal_io_error, IoBufferMut, MaybeFatalIo, VirtualFile};
+use crate::virtual_file::{on_fatal_io_error, MaybeFatalIo, VirtualFile};
 use crate::TEMP_FILE_SUFFIX;
 use remote_storage::{
     DownloadError, DownloadKind, DownloadOpts, GenericRemoteStorage, ListingMode, RemotePath,
@@ -150,7 +149,7 @@ async fn download_object<'a>(
     storage: &'a GenericRemoteStorage,
     src_path: &RemotePath,
     dst_path: &Utf8PathBuf,
-    gate: &utils::sync::gate::Gate,
+    #[cfg_attr(target_os = "macos", allow(unused_variables))] gate: &utils::sync::gate::Gate,
     cancel: &CancellationToken,
     #[cfg_attr(target_os = "macos", allow(unused_variables))] ctx: &RequestContext,
 ) -> Result<u64, DownloadError> {
@@ -209,6 +208,8 @@ async fn download_object<'a>(
         #[cfg(target_os = "linux")]
         crate::virtual_file::io_engine::IoEngine::TokioEpollUring => {
             use crate::virtual_file::owned_buffers_io;
+            use crate::virtual_file::IoBufferMut;
+            use std::sync::Arc;
             async {
                 let destination_file = Arc::new(
                     VirtualFile::create(dst_path, ctx)

pageserver/src/virtual_file/owned_buffers_io/write.rs

@@ -132,6 +132,7 @@ where
             .expect("must not use after we returned an error")
     }
 
+    #[cfg_attr(target_os = "macos", allow(dead_code))]
     pub async fn write_buffered_borrowed(
         &mut self,
         chunk: &[u8],