netfilter: nf_tables: Add meta expression key for bridge interface name

NFT_META_BRI_IIFNAME to get packet input bridge interface name
NFT_META_BRI_OIFNAME to get packet output bridge interface name

Such meta key are accessible only through NFPROTO_BRIDGE family, on a
dedicated nft meta module: nft_meta_bridge.

Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>