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 <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: Tejun Heo <tj@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>