netfilter: ipset: make ip_set_get_ip*_port to use skb_network_offset

All the ipset functions respect skb->network_header value,
except for ip_set_get_ip4_port() & ip_set_get_ip6_port(). The
functions should use skb_network_offset() to get the transport
header offset.

Signed-off-by: Alexander Drozdov <al.drozdov@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>