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: Glauber Costa <[e-mail]>
CC: KAMEZAWA Hiroyuki <[e-mail]>
CC: Johannes Weiner <[e-mail]>
CC: Michal Hocko <[e-mail]>
CC: Tejun Heo <[e-mail]>
CC: Li Zefan <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/res_counter.h

···
 	unsigned long flags;
 
 	spin_lock_irqsave(&cnt->lock, flags);
-	margin = cnt->limit - cnt->usage;
+	if (cnt->limit > cnt->usage)
+		margin = cnt->limit - cnt->usage;
+	else
+		margin = 0;
 	spin_unlock_irqrestore(&cnt->lock, flags);
 	return margin;
 }