sfc: support GRE TSO on EF100

We can treat SKB_GSO_GRE almost exactly the same as UDP tunnels, except
 that we don't want to edit the outer UDP len (as there isn't one).
For SKB_GSO_GRE_CSUM, we have to use GSO_PARTIAL as the device doesn't
 support offload of non-UDP outer L4 checksums.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Martin Habets <mhabets@solarflare.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>