cgroup/cpuset: Don't track # of local child partitions

cgroup/cpuset: Don't track # of local child partitions

The cpuset structure has a nr_subparts field which tracks the number
of child local partitions underneath a particular cpuset. Right now,
nr_subparts is only used in partition_is_populated() to avoid iteration
of child cpusets if the condition is right. So by always performing the
child iteration, we can avoid tracking the number of child partitions
and simplify the code a bit.

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