FROMGIT: mm: memcg: don't periodically flush stats when memcg is disabled

The root memcg is onlined even when memcg is disabled.  When it's onlined
a 2 second periodic stat flush is started, but no stat flushing is
required when memcg is disabled because there can be no child memcgs.
Most calls to flush memcg stats are avoided when memcg is disabled as a
result of the mem_cgroup_disabled check added in 7d7ef0a4 ("mm: memcg:
restore subtree stats flushing"), but the periodic flushing started in
mem_cgroup_css_online is not.  Skip it.

Link: https://lkml.kernel.org/r/20240126211927.1171338-1-tjmercier@google.com


Fixes: aa48e47e ("memcg: infrastructure to flush memcg stats")
Change-Id: Iae6aeb3091d349898ea4987a784a971d9b3c97f7
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Chris Li <chrisl@kernel.org>
Reported-by: Minchan Kim <minchan@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 7e9bccbe57812f888f51d46d7cdbc6327eee24f3
 https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/


 mm-unstable)
Signed-off-by: T.J. Mercier <tjmercier@google.com>