mm/list_lru: code clean up for reparenting

mm/list_lru: code clean up for reparenting

No feature change, just change of code structure and fix comment.

The list lrus are not empty until memcg_reparent_list_lru_node() calls are
all done, so the comments in memcg_offline_kmem were slightly inaccurate.

Link: https://lkml.kernel.org/r/20240925171020.32142-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>