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>

include/linux/netfilter/nfnetlink.h

@@ -11,6 +11,7 @@ struct nfnl_info {
 	struct net		*net;
 	struct sock		*sk;
 	const struct nlmsghdr	*nlh;
+	const struct nfgenmsg	*nfmsg;
 	struct netlink_ext_ack	*extack;
 };