memcg: unify memcg stat flushing

The memcg stats can be flushed in multiple context and potentially in
parallel too.  For example multiple parallel user space readers for
memcg stats will contend on the rstat locks with each other.  There is
no need for that.  We just need one flusher and everyone else can
benefit.

In addition after aa48e47e ("memcg: infrastructure to flush memcg
stats") the kernel periodically flush the memcg stats from the root, so,
the other flushers will potentially have much less work to do.

Link: https://lkml.kernel.org/r/20211001190040.48086-2-shakeelb@google.com


Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: "Michal Koutný" <mkoutny@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>