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>

include/linux/if_ether.h

@@ -56,6 +56,7 @@
 #define ETH_P_DIAG      0x6005          /* DEC Diagnostics              */
 #define ETH_P_CUST      0x6006          /* DEC Customer use             */
 #define ETH_P_SCA       0x6007          /* DEC Systems Comms Arch       */
+#define ETH_P_TEB	0x6558		/* Trans Ether Bridging		*/
 #define ETH_P_RARP      0x8035		/* Reverse Addr Res packet	*/
 #define ETH_P_ATALK	0x809B		/* Appletalk DDP		*/
 #define ETH_P_AARP	0x80F3		/* Appletalk AARP		*/