netfilter: nfnetlink: add struct nfgenmsg to struct nfnl_info and use it

Update the nfnl_info structure to add a pointer to the nfnetlink header.
This simplifies the existing codebase since this header is usually
accessed. Update existing clients to use this new field.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>