mlx4_en: Allow communication between functions on same host

To enable internal loopback, always fill DMAC in control segment
when transmitting the packet, once this is done, the packet is subject
for loopback for if the DMAC mathces one of the multicast/unicast addresses
registered on the physical port.
In receive path if source MAC is our own MAC and we are not in selftest,
or not in force LB mode - drop this packet.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>