mm: memcontrol: remove the kmem states

Now the kmem states is only used to indicate whether the kmem is
offline.  However, we can set ->kmemcg_id to -1 to indicate whether the
kmem is offline.  Finally, we can remove the kmem states to simplify the
code.

Link: https://lkml.kernel.org/r/20211025125259.56624-1-songmuchun@bytedance.com


Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>