cgroup: replace css_set walking populated test with testing cgrp->nr_populated_csets

Implement trivial cgroup_has_tasks() which tests whether
cgrp->nr_populated_csets is zero and replace the explicit local
populated test in cgroup_subtree_control().  This simplifies the code
and cgroup_has_tasks() will be used in more places later.

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