memcg: don't call memcg_update_all_caches if new cache id fits

memcg_update_all_caches grows arrays of per-memcg caches, so we only need
to call it when memcg_limited_groups_array_size is increased.  However,
currently we invoke it each time a new kmem-active memory cgroup is
created.  Then it just iterates over all slab_caches and does nothing
(memcg_update_cache_size returns immediately).

This patch fixes this insanity.  In the meantime it moves the code dealing
with id allocations to separate functions, memcg_alloc_cache_id and
memcg_free_cache_id.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>