efi: add func run_sync() after apply_diff()

Get pointer according to https://github.com/coreos/bootupd/pull/855/files#r2045400178
```
Run a sync after apply_diff rather than immediately unmounting
and only umount after the sync is done.
```

Author: HuijingHei

src/efi.rs

@@ -291,6 +291,7 @@ impl Component for Efi {
             log::trace!("applying adoption diff: {}", &diff);
             filetree::apply_diff(&updated, &esp, &diff, None)
                 .context("applying filesystem changes")?;
+            run_sync()?;
             self.unmount().context("unmount after adopt")?;
         }
         Ok(InstalledContent {
@@ -380,6 +381,7 @@ impl Component for Efi {
             log::trace!("applying diff: {}", &diff);
             filetree::apply_diff(&updated, &destdir, &diff, None)
                 .context("applying filesystem changes")?;
+            run_sync()?;
             self.unmount().context("unmount after update")?;
         }
         let adopted_from = None;
@@ -497,6 +499,15 @@ impl Drop for Efi {
     }
 }
 
+// Run sync after apply_diff()
+fn run_sync() -> Result<()> {
+    Command::new("sync")
+        .run()
+        .with_context(|| format!("Failed to run sync"))?;
+    log::trace!("Sync");
+    Ok(())
+}
+
 fn validate_esp_fstype(dir: &openat::Dir) -> Result<()> {
     let dir = unsafe { BorrowedFd::borrow_raw(dir.as_raw_fd()) };
     let stat = rustix::fs::fstatfs(&dir)?;