netfilter: nf_tables: add xfrm expression

supports fetching saddr/daddr of tunnel mode states, request id and spi.
If direction is 'in', use inbound skb secpath, else dst->xfrm.

Joint work with Máté Eckl.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>