memcg: make memcg's life cycle the same as cgroup

(I'll be off from my office soon, and I won't be responsive in the following
3 days.)

I'm working on converting memcg to use cgroup->id, and then we can kill css_id.

Now memcg has its own refcnt, so when a cgroup is destroyed, the memcg can
still be alive. This patchset converts memcg to always use css_get/put, so
memcg will have the same life cycle as its corresponding cgroup, and then
it's always safe for memcg to use cgroup->id.

The historical reason that memcg didn't use css_get in some cases, is that
cgroup couldn't be removed if there're still css refs. The situation has
changed so that rmdir a cgroup will succeed regardless css refs, but won't
be freed until css refs goes down to 0.

This is an early post, and it's NOT TESTED. I just want to see if the changes
are fine in general.

btw, after this patchset I think we don't need to free memcg via RCU, because
cgroup is already freed in RCU callback.

Note this patchset is based on a few memcg fixes I sent (but hasn't been
accepted)

--
 kernel/cgroup.c |  10 ++++++++
 mm/memcontrol.c | 129 ++++++++++++++++++++++++++++++++++++++-------------------------------------------------------
 2 files changed, 62 insertions(+), 77 deletions(-)
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