Currently it's using the wrong ETH_P_LOOP type, which is sometimes treated
as packet length instead of ether type (because it's 0x0060).

Use the new ETH_P_LOOPBACK type.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>