memcg-v1: no need for memcg locking for MGLRU

While updating the generation of the folios, MGLRU requires that the
folio's memcg association remains stable.  With the charge migration
deprecated, there is no need for MGLRU to acquire locks to keep the folio
and memcg association stable.

[yuzhao@google.com: remove !rcu_read_lock_held() assertion]
  Link: https://lkml.kernel.org/r/ZykEtcHrQRq-KrBC@google.com
  Link: https://syzkaller.appspot.com/bug?extid=24f45b8beab9788e467e
  Link: https://lore.kernel.org/lkml/67294349.050a0220.701a.0010.GAE@google.com/
[akpm@linux-foundation.org: remove now-unused local]
[shakeel.butt@linux.dev: folio_rcu() fixup, per Yu Zhao]
  Link: https://lkml.kernel.org/r/iwmabnye3nl4merealrawt3bdvfii2pwavwrddrqpraoveet7h@ezrsdhjwwej7
Link: https://lkml.kernel.org/r/20241025012304.2473312-6-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>