netfilter: nft_exthdr: add TCP option matching

This patch implements the kernel side of the TCP option patch.

Signed-off-by: Manuel Messner <mm@skelett.io>
Reviewed-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>