s390/qeth: don't cache MAC addresses for multicast IPs

Instead of storing the multicast-mapped MAC address in an IP address
object, just calculate the MAC address when actually building a cmd
for the IP address.

While at it, also clean up some rather verbose copying of IP addresses.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>