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>