gre: Add Transparent Ethernet Bridging

This patch adds support for Ethernet over GRE encapsulation.
This is exposed to user-space with a new link type of "gretap"
instead of "gre".  It will create an ARPHRD_ETHER device in
lieu of the usual ARPHRD_IPGRE.

Note that to preserver backwards compatibility all Transparent
Ethernet Bridging packets are passed to an ARPHRD_IPGRE tunnel
if its key matches and there is no ARPHRD_ETHER device whose
key matches more closely.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>