mm: memcontrol: flush percpu slab vmstats on kmem offlining

I've noticed that the "slab" value in memory.stat is sometimes 0, even
if some children memory cgroups have a non-zero "slab" value.  The
following investigation showed that this is the result of the kmem_cache
reparenting in combination with the per-cpu batching of slab vmstats.

At the offlining some vmstat value may leave in the percpu cache, not
being propagated upwards by the cgroup hierarchy.  It means that stats
on ancestor levels are lower than actual.  Later when slab pages are
released, the precise number of pages is substracted on the parent
level, making the value negative.  We don't show negative values, 0 is
printed instead.

To fix this issue, let's flush percpu slab memcg and lruvec stats on
memcg offlining.  This guarantees that numbers on all ancestor levels
are accurate and match the actual number of outstanding slab pages.

Link: http://lkml.kernel.org/r/20190819202338.363363-3-guro@fb.com
Fixes: fb2f2b0a

 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal")
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>