netfilter: nft_meta_bridge: introduce NFT_META_BRI_IIFHWADDR support

Expose the input bridge interface ethernet address so it can be used to
redirect the packet to the receiving physical device for processing.

Tested with nft command line tool.

table bridge nat {
	chain PREROUTING {
		type filter hook prerouting priority 0; policy accept;
		ether daddr de:ad:00:00:be:ef meta pkttype set host ether daddr set meta ibrhwdr accept
	}
}

Joint work with Pablo Neira.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Florian Westphal <fw@strlen.de>