netfilter: nat: remove module reference counting from NAT protocols

The only remaining user of NAT protocol module reference counting is NAT
ctnetlink support. Since this is a fairly short sequence of code, convert
over to use RCU and remove module reference counting.

Module unregistration is already protected by RCU using synchronize_rcu(),
so no further changes are necessary.

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