memcg: simplify and inline __mem_cgroup_from_kmem

memcg: simplify and inline __mem_cgroup_from_kmem

Before the previous patch ("memcg: unify slab and other kmem pages
charging"), __mem_cgroup_from_kmem had to handle two types of kmem - slab
pages and pages allocated with alloc_kmem_pages - memcg in the page
struct.  Now we can unify it.  Since after it, this function becomes tiny
we can fold it into mem_cgroup_from_kmem.

[[email protected]: move mem_cgroup_from_kmem into list_lru.c]
Signed-off-by: Vladimir Davydov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>