getDiagnosticsData fails: www-data cannot mkdir /var/www/html/userdata on SSVM (Permission denied)

# problem

Retrieving diagnostics from a system VM fails. The archive is collected on the target VM, but the async job then errors and no download URL is produced:

```
Unable to create a link for entity at diagnostics//diagnostics_files_.zip on ssvm, Error in creating directory =mkdir: cannot create directory '/var/www/html/userdata//': Permission denied
```

Expected: the job succeeds and returns a usable download URL.

# versions

- CloudStack 4.22.1.0 (management and agents)
- System VMs from the stock `systemvm-kvm-4.22.0` template, not customized
- KVM hypervisors, NFS secondary storage

# The steps to reproduce the bug

1. Run `getDiagnosticsData` against a running system VM, e.g. `cmk get diagnosticsdata targetid= files=/var/log/cloud.log`.
2. Wait for the async job to complete.
3. The job fails with errorcode 530 and the error above.

# What to do about it?

The download directory is created as the `www-data` user ([UploadManagerImpl.java#L277-L291](https://github.com/apache/cloudstack/blob/4.22/services/secondary-storage/server/src/main/java/org/apache/cloudstack/storage/template/UploadManagerImpl.java#L277-L291)), but the docroot on the SSVM is not writable by `www-data`. Shipping `/var/www/html/userdata` owned by `www-data` in the system VM template, or creating it as root and chowning, would fix it. `extractVolume`/`extractTemplate` downloads use the same directory and may be affected as well; only the diagnostics case is verified.

Workaround on a running SSVM: `mkdir -p /var/www/html/userdata && chown www-data:www-data /var/www/html/userdata` (lost on SSVM recreation).

Bắt đầu với các dòng 277-291 của UploadManagerImpl.java và tái hiện getDiagnosticsData trên một system VM đang chạy bằng lệnh trong issue. Kiểm tra cách template SSVM tạo và thiết lập quyền cho /var/www/html/userdata, bao gồm các đường dẫn extractVolume và extractTemplate liên quan. Hoàn tất khi async job thành công và trả về một download URL có thể sử dụng mà không cần workaround thủ công.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.