cgroup: Replace the css_set call with cgroup_get

We will release the refcnt of cgroup via cgroup_put, for example
in the cgroup_lock_and_drain_offline function, so replace css_get
with the cgroup_get function for better readability.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>