memcg: cleanup racy sum avoidance code

We used to have per-cpu memcg and lruvec stats and the readers have to
traverse and sum the stats from each cpu.  This summing was racy and may
expose transient negative values.  So, an explicit check was added to
avoid such scenarios.  Now these stats are moved to rstat infrastructure
and are no more per-cpu, so we can remove the fixup for transient negative
values.

Link: https://lkml.kernel.org/r/20210728012243.3369123-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>