fs/resctrl: Refactor rmdir_mondata_subdir_allrdtgrp()

fs/resctrl: Refactor rmdir_mondata_subdir_allrdtgrp()

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

Refactor the SNC case into a helper function to make it easier to add support
for a new telemetry resource.

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