sl[au]b: charge slabs to kmemcg explicitly

sl[au]b: charge slabs to kmemcg explicitly

We have only a few places where we actually want to charge kmem so instead
of intruding into the general page allocation path with __GFP_KMEMCG it's
better to explictly charge kmem there.  All kmem charges will be easier to
follow that way.

This is a step towards removing __GFP_KMEMCG.  It removes __GFP_KMEMCG
from memcg caches' allocflags.  Instead it makes slab allocation path call
memcg_charge_kmem directly getting memcg to charge from the cache's memcg
params.

This also eliminates any possibility of misaccounting an allocation going
from one memcg's cache to another memcg, because now we always charge
slabs against the memcg the cache belongs to.  That's why this patch
removes the big comment to memcg_kmem_get_cache.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>