netfilter: ctnetlink: add full support for SCTP to ctnetlink

This patch adds full support for SCTP to ctnetlink. This includes three
new attributes: state, original vtag and reply vtag.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>