xtables: arp: inhibit -l option so only a fixed 6 bytes length arhln can be used

xtables: arp: inhibit -l option so only a fixed 6 bytes length arhln can be used

This is a temporary workaround mechanism until variable interface
hardware address length can be handled through nftables. This
defaults on the length of EUI-64 mac address, which should be the
most common usage until this is appropriately fixed for all type
of layer 2 addresses.

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