netfilter: nft_meta: add support for slave device ifindex matching

Allow to match on vrf slave ifindex or name.

In case there was no slave interface involved, store 0 in the
destination register just like existing iif/oif matching.

sdif(name) is restricted to the ipv4/ipv6 input and forward hooks,
as it depends on ip(6) stack parsing/storing info in skb->cb[].

Cc: Martin Willi <martin@strongswan.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Shrijeet Mukherjee <shrijeet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>