ip6_gre: Do not allow segmentation offloads GRE_CSUM is enabled with FOU/GUE

This patch addresses the same issue we had for IPv4 where enabling GRE with
an inner checksum cannot be supported with FOU/GUE due to the fact that
they will jump past the GRE header at it is treated like a tunnel header.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>