netlink: nlpacket AttributeMACAddress

netlink: nlpacket AttributeMACAddress

This commits fixes AttributeMACAddress for GREv6.
Error message: info: netlink: link dump failed: Length of MACAddress attribute not supported: 20
Reproducible by adding a GREv6 Tunnel:

auto gre1
iface gre1 inet tunnel
    mode ip6gre
    local 2001:1000:1000:1000::123
    endpoint 2001:1000:1000:2000::123
    address 192.168.123.1/29

In netlink The IFLA_ADDRESS and IFLA_BROADCAST attributes for all interfaces has been a
6-byte MAC address. But the GRE interface uses a 4-byte IP address and GREv6 uses a 16-byte IPv6 address for this
attribute. This patch allows for decoding a 16-byte value as an IP address.