memcg: calculate root usage from global state

Currently, we approximate the root usage by adding the memcg stats for
anon, file, and conditionally swap (for memsw).  To read the memcg stats
we need to invoke an rstat flush.  rstat flushes can be expensive, they
scale with the number of cpus and cgroups on the system.

mem_cgroup_usage() is called by memcg_events()->mem_cgroup_threshold()
with irqs disabled, so such an expensive operation with irqs disabled can
cause problems.

Instead, approximate the root usage from global state.  This is not 100%
accurate, but the root usage has always been ill-defined anyway.

Link: https://lkml.kernel.org/r/20230421174020.2994750-4-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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