Monitoring tools that want to find out which resctrl control and monitor
groups a task belongs to must currently read the "tasks" file in every
group until they locate the process ID.

Add an additional file /proc/{pid}/cpu_resctrl_groups to provide this
information:

1)   res:
     mon:

resctrl is not available.

2)   res:/
     mon:

Task is part of the root resctrl control group, and it is not associated
to any monitor group.

3)  res:/
    mon:mon0

Task is part of the root resctrl control group and monitor group mon0.

4)  res:group0
    mon:

Task is part of resctrl control group group0, and it is not associated
to any monitor group.

5) res:group0
   mon:mon1

Task is part of resctrl control group group0 and monitor group mon1.
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Jinshi Chen <jinshi.chen@intel.com>
Link: https://lkml.kernel.org/r/20200115092851.14761-1-yu.c.chen@intel.com