AlmostFull is not accurate

Bug Report

Please answer these questions before submitting your issue. Thanks!

In init_storage_stats_task, we should use TiFlash's config.

...
        self.background_worker
            .spawn_interval_task(DEFAULT_STORAGE_STATS_INTERVAL, move || {
                let disk_stats = match fs2::statvfs(&store_path) {
                    Err(e) => {
                        error!(
                            "get disk stat for kv store failed";
                            "kv path" => store_path.to_str(),
                            "err" => ?e
                        );
                        return;
                    }
                    Ok(stats) => stats,
                };
...

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)

Start by locating init_storage_stats_task and reviewing the fs2::statvfs call shown in the report. Trace the storage path and TiFlash configuration used there; the work is done when the storage statistics task uses TiFlash's config as requested and the resulting behavior can be verified.