[IPV6]: Fixed the size of the netlink message notified by inet6_rt_notify().

I think the return value of rt6_nlmsg_size() should includes the
amount of RTA_METRICS.

Signed-off-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>