netfilter: masquerade: attach nat extension if not present

Currently the nat extension is always attached as soon as nat module is
loaded.  However, most NAT uses do not need the nat extension anymore.

Prepare to remove the add-nat-by-default by making those places that need
it attach it if its not present yet.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>