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

[ 19c28b1374fb1073a9ec873a6c10bf5f16b10b9d ]

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>