flow_dissector: support L2 GRE

Add support for L2 GRE tunnels, so that RPS can be more effective.

Signed-off-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>