fix(macos): gate no_fsync method and call site for macOS only

The no_fsync feature is specific to macOS HVF framework which uses
a global shared lock. This commit gates the set_no_fsync method
and its call site so they only apply on macOS.

Fixes Linux build failure where no_fsync field doesn't exist
in the Linux Config struct.

Author: RoyLin

src/devices/src/virtio/fs/device.rs

@@ -148,6 +148,7 @@ impl Fs {
         self.passthrough_cfg.export_fsid
     }
 
+    #[cfg(target_os = "macos")]
     pub fn set_no_fsync(&mut self, no_fsync: bool) {
         self.passthrough_cfg.no_fsync = no_fsync;
     }

src/vmm/src/builder.rs

@@ -3668,6 +3668,7 @@ fn attach_fs_devices(
         let id = format!("{}{}", String::from(fs.lock().unwrap().id()), i);
 
         // Set no_fsync option if enabled
+        #[cfg(target_os = "macos")]
         if config.no_fsync {
             fs.lock().unwrap().set_no_fsync(true);
         }