bridge: reserve space for IFLA_BRPORT_FAST_LEAVE

The bridge multicast fast leave feature was added sufficient space
was not reserved in the netlink message. This means the flag may be
lost in netlink events and results of queries.

Found by observation while looking up some netlink stuff for discussion with Vlad.
Problem introduced by commit c2d3babf
Author: David S. Miller <davem@davemloft.net>
Date:   Wed Dec 5 16:24:45 2012 -0500

    bridge: implement multicast fast leave

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>