bridge: reserve space for IFLA_BRPORT_FAST_LEAVE

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 008fd7156c4c4a17c7d10a9c1d44ca4c3f7ec6ff
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>