cgroup/cpuset: Make cpuset_attach_task() skip subpartitions CPUs for top_cpuset

It is found that attaching a task to the top_cpuset does not currently
ignore CPUs allocated to subpartitions in cpuset_attach_task(). So the
code is changed to fix that.
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cpuset.c

@@ -2535,7 +2535,8 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 	if (cs != &top_cpuset)
 		guarantee_online_cpus(task, cpus_attach);
 	else
-		cpumask_copy(cpus_attach, task_cpu_possible_mask(task));
+		cpumask_andnot(cpus_attach, task_cpu_possible_mask(task),
+			       cs->subparts_cpus);
 	/*
 	 * can_attach beforehand should guarantee that this doesn't
 	 * fail.  TODO: have a better way to handle failure here