cgroup: minor tweak for logic to get cgroup css

We could only handle the case that css exists
and css_try_get_online() fails.
Signed-off-by: Peng Wang <rocking@whu.edu.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -488,7 +488,7 @@ static struct cgroup_subsys_state *cgroup_tryget_css(struct cgroup *cgrp,
 
 	rcu_read_lock();
 	css = cgroup_css(cgrp, ss);
-	if (!css || !css_tryget_online(css))
+	if (css && !css_tryget_online(css))
 		css = NULL;
 	rcu_read_unlock();