fix: SSVM entity download URL creation: ensure userdata base dir exists and is writable by www-data

[waterWang]

Problem

When handleCreateEntityURLCommand runs on the SSVM (Secondary Storage VM) to build a download URL for e.g. getDiagnosticsData, it creates /var/www/html/userdata/<uuid>/ as the www-data user via su www-data -c "mkdir -p ...". On stock systemvm templates (systemvm-kvm-4.22.0) the /var/www/html/userdata base directory does not exist, and /var/www/html is not writable by www-data, so the mkdir fails with:

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

This affects getDiagnosticsData, extractVolume, extractTemplate and any other operation that calls CreateEntityDownloadURLCommand.

Fix

Create the base extract directory (/var/www/html/userdata/) and set its ownership to www-data:www-data before the privilege-dropped su www-data call that creates the per-entity subdirectory. This self-heals the SSVM on first use -- the directory is created by the root process that owns the service, then handed over to www-data for the downstream Apache docroot operations.

Changes

UploadManagerImpl.java -- handleCreateEntityURLCommand: add mkdir -p BASE_EXTRACT_PATH && chown www-data:www-data BASE_EXTRACT_PATH before the existing su www-data subdirectory creation.

+12 / -1 lines.

Closes #13959

[DaanHoogland]

@waterWang , please rebase on 4.22