GRE: Add support for GRO/GSO of IPv6 GRE traffic

Since GRE doesn't really care about L3 protocol we can support IPv4 and
IPv6 using the same offloads.  With that being the case we can add a call
to register the offloads for IPv6 as a part of our GRE offload
initialization.

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