memcg: use helper macro FLUSH_TIME

Use helper macro FLUSH_TIME to indicate the flush time to improve the
readability a bit. No functional change intended.

Link: https://lkml.kernel.org/r/20230603072116.1101690-1-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -5428,7 +5428,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 
 	if (unlikely(mem_cgroup_is_root(memcg)))
 		queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
-				   2UL*HZ);
+				   FLUSH_TIME);
 	lru_gen_online_memcg(memcg);
 	return 0;
 offline_kmem: