openvswitch: load and reference the NAT helper.

This improves the original commit 17c357ef ("openvswitch: load
NAT helper") where it unconditionally tries to load the module for
every flow using NAT, so not efficient when loading multiple flows.
It also doesn't hold any references to the NAT module while the
flow is active.

This change fixes those problems. It will try to load the module
only if it's not present. It grabs a reference to the NAT module
and holds it while the flow is active. Finally, an error message
shows up if either actions above fails.

Fixes: 17c357ef

 ("openvswitch: load NAT helper")
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>