cgroup/cpuset: introduce cpuset-v1.c

This patch introduces the cgroup/cpuset-v1.c source file which will be
used for all legacy (cgroup v1) cpuset cgroup code. It also introduces
cgroup/cpuset-internal.h to keep declarations shared between
cgroup/cpuset.c and cpuset/cpuset-v1.c.

As of now, let's compile it if CONFIG_CPUSET is set. Later on it can be
switched to use a separate config option, so that the legacy code won't be
compiled if not required.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>