cgroup: fix lockdep warning in debug controller

The debug controller grabs cgroup_mutex from interface file show
functions which can deadlock and triggers lockdep warnings.  Fix it by
using cgroup_kn_lock_live()/cgroup_kn_unlock() instead.

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