cgroup: reorganize include/linux/cgroup.h

From c4d440938b5e2015c70594fe6666a099c844f929 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 13 May 2015 16:21:40 -0400

Over time, cgroup.h grew organically and doesn't have much logical
structure at this point.  Separation of cgroup-defs.h in the previous
patch gives us a good chance for reorganizing cgroup.h as changes to
the header are likely to cause conflicts anyway.

This patch reorganizes cgroup.h so that it has consistent logical
grouping.

This is pure reorganization.

v2: Relocating #ifdef CONFIG_CGROUPS caused build failure when cgroup
    is disabled.  Dropped.
Signed-off-by: Tejun Heo <tj@kernel.org>