netfilter: add helper function to set up the nfnetlink header and use it

[ Upstream commit 19c28b13 ]

This patch adds a helper function to set up the netlink and nfnetlink headers.
Update existing codebase to use it.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>