bridge: fix mdb info leaks

The bridging code discloses heap and stack bytes via the RTM_GETMDB
netlink interface and via the notify messages send to group RTNLGRP_MDB
afer a successful add/del.

Fix both cases by initializing all unset members/padding bytes with
memset(0).

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int br_mdb_add(struct sk_buff *skb, struct nlmsghdr *nlh, void *arg)
{
	struct net *net = sock_net(skb->sk);
	struct br_mdb_entry *entry;
	struct net_device *dev;
	struct net_bridge *br;
	int err;

	err = br_mdb_parse(skb, nlh, &dev, &entry);
	if (err < 0)
		return err;

	br = netdev_priv(dev);

	err = __br_mdb_add(net, br, entry);
	if (!err)
		__br_mdb_notify(dev, entry, RTM_NEWMDB);
	return err;
}

static int br_mdb_add(struct sk_buff *skb, struct nlmsghdr *nlh, void *arg)
{
	struct net *net = sock_net(skb->sk);
	struct br_mdb_entry *entry;
	struct net_device *dev;
	struct net_bridge *br;
	int err;

	err = br_mdb_parse(skb, nlh, &dev, &entry);
	if (err < 0)
		return err;

	br = netdev_priv(dev);

	err = __br_mdb_add(net, br, entry);
	if (!err)
		__br_mdb_notify(dev, entry, RTM_NEWMDB);
	return err;
}
