memcg: remove redundant code in mem_cgroup_force_empty_write()

vfs guarantees the cgroup won't be destroyed, so it's redundant to get a
css reference.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4990,18 +4990,12 @@ static int mem_cgroup_force_empty_write(struct cgroup_subsys_state *css,
 					unsigned int event)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-	int ret;
 
 	if (mem_cgroup_is_root(memcg))
 		return -EINVAL;
-	css_get(&memcg->css);
-	ret = mem_cgroup_force_empty(memcg);
-	css_put(&memcg->css);
-
-	return ret;
+	return mem_cgroup_force_empty(memcg);
 }
 
-
 static u64 mem_cgroup_hierarchy_read(struct cgroup_subsys_state *css,
 				     struct cftype *cft)
 {