mm: memcontrol: replace the loop with a list_for_each_entry()

mm: memcontrol: replace the loop with a list_for_each_entry()

The rule of list walk has gone since commit 301cf26d6261
("mm/memcontrol: allow to uncharge page without using page->lru field")

So remove the strange comment and replace the loop with a
list_for_each_entry().

There is only one caller of the uncharge_list().  So just fold it into
mem_cgroup_uncharge_list() and remove it.

Link: https://lkml.kernel.org/r/20210204163055.56080-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>