src: support for arp sender and target ethernet and IPv4 addresses

src: support for arp sender and target ethernet and IPv4 addresses

 # nft add table arp x
 # nft add chain arp x y { type filter hook input priority 0\; }
 # nft add rule arp x y arp saddr ip 192.168.2.1 counter

Testing this:

 # ip neigh flush dev eth0
 # ping 8.8.8.8
 # nft list ruleset
 table arp x {
        chain y {
                type filter hook input priority filter; policy accept;
                arp saddr ip 192.168.2.1 counter packets 1 bytes 46
        }
 }

You can also specify hardware sender address, eg.

 # nft add rule arp x y arp saddr ether aa:bb:cc:aa:bb:cc drop counter

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