[IPV6]: Sum real space for RTAs.

This patch fixes RTNLGRP_IPV6_IFINFO netlink notifications.  Issue
pointed out by Patrick McHardy <kaber@trash.net>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>