cgroup: disallow debug controller on the default hierarchy

The debug controller, as its name suggests, exposes cgroup core
internals to userland to aid debugging.  Unfortunately, except for the
name, there's no provision to prevent its usage in production
configurations and the controller is widely enabled and mounted
leaking internal details to userland.  Like most other debug
information, the information exposed by debug isn't interesting even
for debugging itself once the related parts are working reliably.

This controller has no reason for existing.  This patch implements
cgrp_dfl_root_inhibit_ss_mask which can suppress specific subsystems
on the default hierarchy and adds the debug subsystem to it so that it
can be gradually deprecated as usages move towards the unified
hierarchy.

Signed-off-by: Tejun Heo <tj@kernel.org>