cgroup: make sure memcg margin is 0 when over limit

For the memcg sock code, we'll need to register allocations
that are temporarily over limit. Let's make sure that margin
is 0 in this case.

I am keeping this as a separate patch, so that if any weirdness
interaction appears in the future, we can now exactly what caused
it.

Suggested by Johannes Weiner
Signed-off-by: NGlauber Costa <glommer@parallels.com>
CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Michal Hocko <mhocko@suse.cz>
CC: Tejun Heo <tj@kernel.org>
CC: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/res_counter.h

@@ -142,7 +142,10 @@ static inline unsigned long long res_counter_margin(struct res_counter *cnt)
 	unsigned long flags;
 
 	spin_lock_irqsave(&cnt->lock, flags);
+	if (cnt->limit > cnt->usage)
 		margin = cnt->limit - cnt->usage;
+	else
+		margin = 0;
 	spin_unlock_irqrestore(&cnt->lock, flags);
 	return margin;
 }