mm: use for_each_online_node and node_online instead of open coding

Use more generic functions to deal with issues related to online nodes. 
The changes will make the code simplified.

Link: https://lkml.kernel.org/r/20220429030218.644635-1-liupeng256@huawei.com


Signed-off-by: Peng Liu <liupeng256@huawei.com>
Suggested-by: Davidlohr Bueso <dave@stgolabs.net>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>