cpuset: rename @cont to @cgrp

Cont is short for container. control group was named process container
at first, but then people found container already has a meaning in
linux kernel.

Clean up the leftover variable name @cont.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>