[RTNETLINK]: Fix IFLA_ADDRESS handling.

The ->set_mac_address handlers expect a pointer to a
sockaddr which contains the MAC address, whereas
IFLA_ADDRESS provides just the MAC address itself.

So whip up a sockaddr to wrap around the netlink
attribute for the ->set_mac_address call.

Signed-off-by: David S. Miller <davem@davemloft.net>