netfilter: nf_nat: use per-conntrack locking for sequence number adjustments

Get rid of the global lock and use per-conntrack locks for protecting the
sequencen number adjustment data. Additionally saves one lock/unlock
operation for every TCP packet.

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