i40e: fix offload of GRE tunnels

The driver still was not offloading TSO on GRE tunnels because
it forgot to set the GSO_GRE flag, causing lots of retransmits.

This fixes generic GRE traffic (like a tunnel added like below)
whereas before it would get 1Gb/s or less, now on a 10G adapter
it gets 8.7Gb/s.

ip ad ad 11.1.0.2/24 dev ens2f0
ip l set ens2f0 up
ip link add gre2 type gretap remote 11.1.0.1 local 11.1.0.2 dev ens2f0
ip l set gre2 up
ip ad ad 192.168.124.2/24 dev gre2
ping 192.168.124.1
netperf -H 192.168.124.1

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>