netfilter: h323: merge nat hook pointers into one

sparse complains about incorrect rcu usage.

Code uses the correct rcu access primitives, but the function pointers
lack rcu annotations.

Collapse all of them into a single structure, then annotate the pointer.

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