[NETFILTER]: nf_conntrack_netlink: add support to related connections

This patch adds support to relate a connection to an existing master
connection. This patch is used by conntrackd to correctly replicate
related connections.

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>