app/testpmd: enable TCP/IPv4 VxLAN and GRE GSO

app/testpmd: enable TCP/IPv4 VxLAN and GRE GSO

This patch adds GSO support to the csum forwarding engine. Oversized
packets transmitted over a GSO-enabled port will undergo segmentation
(with the exception of packet-types unsupported by the GSO library).
GSO support is disabled by default.

GSO support may be toggled on a per-port basis, using the command:

        "set port <port_id> gso on|off"

The maximum packet length (including the packet header and payload) for
GSO segments may be set with the command:

        "set gso segsz <length>"

Show GSO configuration for a given port with the command:

	"show port <port_id> gso"

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>