geneve: avoid indirect calls in GRO path, when possible

In the most common setups, the geneve tunnels use an inner
ethernet encapsulation. In the GRO path, when such condition is
true, we can call directly the relevant GRO helper and avoid
a few indirect calls.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>