cgroup/cpuset: Keep track of CPUs in isolated partitions

Add a new internal isolated_cpus mask to keep track of the CPUs that are in
isolated partitions. Expose that new cpumask as a new root-only control file
".cpuset.cpus.isolated".

tj: Updated patch description to reflect dropping __DEBUG__ prefix.

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