ibmvnic: Include header descriptor support for ARP packets

In recent tests with new adapters, it was discovered that ARP
packets were not being properly processed. This patch adds
support for ARP packet headers to be passed to backing adapters,
if necessary.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>