cgroup: add cgroup.stat interface with basic hierarchy stats

A cgroup can consume resources even after being deleted by a user.
For example, writing back dirty pages should be accounted and
limited, despite the corresponding cgroup might contain no processes
and being deleted by a user.

In the current implementation a cgroup can remain in such "dying" state
for an undefined amount of time. For instance, if a memory cgroup
contains a pge, mlocked by a process belonging to an other cgroup.

Although the lifecycle of a dying cgroup is out of user's control,
it's important to have some insight of what's going on under the hood.

In particular, it's handy to have a counter which will allow
to detect css leaks.

To solve this problem, add a cgroup.stat interface to
the base cgroup control files with the following metrics:

nr_descendants		total number of visible descendant cgroups
nr_dying_descendants	total number of dying descendant cgroups

Signed-off-by: Roman Gushchin <[email protected]>
Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]

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