memcg: restructure mem_cgroup_can_attach()

static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,

				 struct cgroup_taskset *tset)

{

	struct task_struct *p = cgroup_taskset_first(tset);

	int ret = 0;

	struct mem_cgroup *memcg = mem_cgroup_from_css(css);

	struct mem_cgroup *from;

	struct task_struct *p;

	struct mm_struct *mm;

	unsigned long move_flags;

	int ret = 0;

	/*

	 * We are now commited to this value whatever it is. Changes in this

	 * So we need to save it, and keep it going.

	 */

	move_flags = READ_ONCE(memcg->move_charge_at_immigrate);

	if (move_flags) {

		struct mm_struct *mm;

		struct mem_cgroup *from = mem_cgroup_from_task(p);

	if (!move_flags)

		return 0;

	p = cgroup_taskset_first(tset);

	from = mem_cgroup_from_task(p);

	VM_BUG_ON(from == memcg);

			mem_cgroup_clear_mc();

	}

	mmput(mm);

	}

	return ret;

}