memcg: zap __memcg_{charge,uncharge}_slab

They are simple wrappers around memcg_{charge,uncharge}_kmem, so let's
zap them and call these functions directly.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>