fs/resctrl: Refactor mkdir_mondata_subdir()

fs/resctrl: Refactor mkdir_mondata_subdir()

Population of a monitor group's mon_data directory is unreasonably complicated
because of the support for Sub-NUMA Cluster (SNC) mode.

Split out the SNC code into a helper function to make it easier to add support
for a new telemetry resource.

Move all the duplicated code to make and set owner of domain directories into
the mon_add_all_files() helper and rename to _mkdir_mondata_subdir().

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/20251217172121.12030-1-tony.luck@intel.com