app/testpmd: enable TCP/IPv4 GRO

app/testpmd: enable TCP/IPv4 GRO

This patch enables TCP/IPv4 GRO library in csum forwarding engine.
By default, GRO is turned off. Users can use command "gro (on|off)
(port_id)" to enable or disable GRO for a given port. If a port is
enabled GRO, all TCP/IPv4 packets received from the port are performed
GRO. Besides, users can set max flow number and packets number per-flow
by command "gro set (max_flow_num) (max_item_num_per_flow) (port_id)".

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>