memcg: move mem_cgroup_usage memcontrol-v1.c

memcg: move mem_cgroup_usage memcontrol-v1.c

Patch series "memcg cleanups", v3.

Two code moves/removals with no behavior change.

This patch (of 2):

Currently, mem_cgroup_usage is only used for v1, just move it to
memcontrol-v1.c

Link: https://lkml.kernel.org/r/20251211013019.2080004-1-chenridong@huaweicloud.com
Link: https://lkml.kernel.org/r/20251211013019.2080004-2-chenridong@huaweicloud.com
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Koutný <mkoutny@suse.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lu Jialin <lujialin4@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>