ipvs: Add destination address family to netlink interface

This is necessary to support heterogeneous pools.  For example, if you have
an ipv6 addressed network, you'll want to be able to forward ipv4 traffic
into it.

This patch enforces that destination address family is the same as service
family, as none of the forwarding mechanisms support anything else.

For the old setsockopt mechanism, we simply set the dest address family to
AF_INET as we do with the service.

Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>