gro: Allow tunnel stacking in the case of FOU/GUE

This patch should fix the issues seen with a recent fix to prevent
tunnel-in-tunnel frames from being generated with GRO.  The fix itself is
correct for now as long as we do not add any devices that support
NETIF_F_GSO_GRE_CSUM.  When such a device is added it could have the
potential to mess things up due to the fact that the outer transport header
points to the outer UDP header and not the GRE header as would be expected.

Fixes: fac8e0f5 ("tunnels: Don't apply GRO to multiple layers of encapsulation.")
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>