Resctrl: Add mon group support.

[Creatone]

RDT Monitoring features:

• CMT ( Cache Monitoring Technology ) supports monitoring of the last-level cache (LLC) occupancy
  for each running thread simultaneously.
• MBM ( Memory Bandwidth Monitoring ) supports monitoring of total and local memory bandwidth
  for each running thread simultaneously.

On a system with RDT monitoring features, the root directory, and other top levels
directories contain a folder named "mon_groups" in which additional
"MON" groups can be created to monitor subsets of tasks from a "CTRL_MON" group that is their ancestor.

There is a limitation of the number of "CTRL_MON" groups.
So in case when there are so many containers and as a user, you only want to use monitoring features, it's easy to reach this limit.
That's why "MON" groups are introduced.

This enhancement should add:

• Basic "MON" groups support - Add mon groups for resctrl. #2523
  Support creating a container with only RDT monitoring features (the "MON" group).

• Dynamically switching between group types ("MON" and "CTRL_MON") after runc update.
  For example:
  In case when as a user, you want to update a container which only uses monitoring features, with other RDT features,
  dynamically switch from "MON" to "CTRL_MON" group (create new group, move tasks, clean). And the other way.

[marquiz]

An effort to simplify and clarify resctrl monitoring:
opencontainers/runtime-spec#1287