[NETFILTER] ctnetlink: add support to change protocol info

This patch add support to change the state of the private protocol
information via conntrack_netlink.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>