fs/resctrl: Add missing return value descriptions

fs/resctrl: Add missing return value descriptions

Using the stricter "./tools/docs/kernel-doc -Wall -v" to verify proper
formatting of documentation comments includes warnings related to return
markup on functions that are omitted during the default verification
checks. This stricter verification reports a couple of missing return
descriptions in resctrl:

    Warning: .../fs/resctrl/rdtgroup.c:1536 No description found for return value of 'rdtgroup_cbm_to_size'
    Warning: .../fs/resctrl/rdtgroup.c:3131 No description found for return value of 'mon_get_kn_priv'
    Warning: .../fs/resctrl/rdtgroup.c:3523 No description found for return value of 'cbm_ensure_valid'
    Warning: .../fs/resctrl/monitor.c:238 No description found for return value of 'resctrl_find_cleanest_closid'

Add the missing return descriptions.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/1c50b9f7c73251c007133590986f127e1af57780.1775576382.git.reinette.chatre@intel.com