cgroup: Make debug cgroup support v2 and thread mode

Besides supporting cgroup v2 and thread mode, the following changes
are also made:
 1) current_* cgroup files now resides only at the root as we don't
    need duplicated files of the same function all over the cgroup
    hierarchy.
 2) The cgroup_css_links_read() function is modified to report
    the number of tasks that are skipped because of overflow.
 3) The number of extra unaccounted references are displayed.
 4) The current_css_set_read() function now prints out the addresses of
    the css'es associated with the current css_set.
 5) A new cgroup_subsys_states file is added to display the css objects
    associated with a cgroup.
 6) A new cgroup_masks file is added to display the various controller
    bit masks in the cgroup.

tj: Dropped thread mode related information for now so that debug
    controller changes aren't blocked on the thread mode.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>