cgroup_freezer: introduce CGROUP_FREEZING_[SELF|PARENT]

Introduce FREEZING_SELF and FREEZING_PARENT and make FREEZING OR of
the two flags.  This is to prepare for full hierarchy support.

freezer_apply_date() is updated such that it can handle setting and
clearing of both flags.  The two flags are also exposed to userland
via read-only files self_freezing and parent_freezing.

Other than the added cgroupfs files, this patch doesn't introduce any
behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>