ibmveth: Detect unsupported packets before sending to the hypervisor

Currently, when ibmveth receive a loopback packet, it reports an
ambiguous error message "tx: h_send_logical_lan failed with rc=-4"
because the hypervisor rejects those types of packets. This fix
detects loopback packet and assures the source packet's MAC address
matches the driver's MAC address before transmitting to the
hypervisor.

Signed-off-by: Cris Forno <cforno12@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>