[NETFILTER]: ctnetlink: add support for NAT sequence adjustments

The combination of NAT and helpers may produce TCP sequence adjustments.
In failover setups, this information needs to be replicated in order to
achieve a successful recovery of mangled, related connections. This patch is
particularly useful for conntrackd, see:

http://people.netfilter.org/pablo/conntrack-tools/



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>