cgroup: return early if it is already on preloaded list

If a cset is already on preloaded list, this means we have already setup
this cset properly for migration.

This patch just relocates the root cgrp lookup which isn't used anyway
when the cset is already on the preloaded list.

[tj@kernel.org: rephrase the commit log]

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>