Advertise the DEV_TX_OFFLOAD_OUTER_IPV4_CKSUM flag in the PMD
features. It means that the i40e PMD supports the offload of outer IP
checksum when transmitting tunneling packet.
Signed-off-by: Jijiang Liu <jijiang.liu@intel.com> Acked-by: Olivier Matz <olivier.matz@6wind.com>