cgroup/cpuset: Include offline CPUs when tasks' cpumasks in top_cpuset are updated

Similar to commit 3fb906e7 ("group/cpuset: Don't filter offline
CPUs in cpuset_cpus_allowed() for top cpuset tasks"), the whole set of
possible CPUs including offline ones should be used for setting cpumasks
for tasks in the top cpuset when a cpuset partition is modified as the
hotplug code won't update cpumasks for tasks in the top cpuset when
CPUs become online or offline.

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