netfilter: nft_meta: add NFT_META_IFTYPE

Generalize NFT_META_IIFTYPE to NFT_META_IFTYPE which allows you to match
on the interface type of the skb->dev field. This field is used by the
netdev family to add an implicit dependency to skip non-ethernet packets
when matching on layer 3 and 4 TCP/IP header fields.

For backward compatibility, add the NFT_META_IIFTYPE alias to
NFT_META_IFTYPE.

Add __NFT_META_IIFTYPE, to be used by userspace in the future to match
specifically on the iiftype.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>