ether: add loopback type ETH_P_LOOPBACK

Per IEEE 802.3*, the correct packet type for loopback 0x9000. There's
already one ETH_P_LOOP 0x0060, which has been there for ages, however it's
plainly wrong as anything that small is considered a length field.

We can't remove it because legacy, so add a new type which corresponds to
the correct id.

http://www.iana.org/assignments/ieee-802-numbers/ieee-802-numbers.xhtml

CC: "David S. Miller" <davem@davemloft.net>
CC: Stefan Richter <stefanr@s5r6.in-berlin.de>
CC: Simon Wunderlich <sw@simonwunderlich.de>
CC: Neil Jerram <Neil.Jerram@metaswitch.com>
CC: Simon Horman <horms@verge.net.au>
CC: Arvid Brodin <Arvid.Brodin@xdin.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>