cgroup: factor out kill_css()

Factor out css ref killing from cgroup_destroy_locked() into
kill_css().  We're gonna add more to the path and the factored out
function will eventually be called from other places too.

While at it, replace open coded percpu_ref_get() with css_get() for
consistency.  This shouldn't cause any functional difference as the
function is not used for root cgroups.

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