netfilter: nft_xfrm: use state family, not hook one

netfilter: nft_xfrm: use state family, not hook one

Eyal says:
  doesn't the use of nft_pf(pkt) in this context limit the matching of
  encapsulated packets to the same family?

  IIUC when an e.g. IPv6-in-IPv4 packet is matched, the nft_pf(pkt) will
  be the decapsulated packet family - IPv6 - whereas the state may be
  IPv4. So this check would not allow matching the 'underlay' address in
  such cases.

  I know this was a limitation in xt_policy. but is this intentional in
  this matcher? or is it possible to use state->props.family when
  validating the match instead of nft_pf(pkt)?

Userspace already tells us which address family it expects to match, so
we can just use the real state family rather than the hook family.
so change it as suggested above.

Reported-by: Eyal Birger <eyal.birger@gmail.com>
Suggested-by: Eyal Birger <eyal.birger@gmail.com>
Fixes: 6c47260250fc6 ("netfilter: nf_tables: add xfrm expression")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>