app/testpmd: perform SW IP checksum for GRO/GSO packets

app/testpmd: perform SW IP checksum for GRO/GSO packets

The GRO/GSO library doesn't re-calculate checksums for
merged/fragmented packets. If users want the packets to
have correct IP checksums, they should select HW IP
checksum calculation for the port which the packets are
transmitted to. But if the port doesn't support HW IP
checksum, users may perform a SW IP checksum.

Fixes: b7091f1dcfbc ("app/testpmd: enable the heavyweight mode TCP/IPv4 GRO")
Fixes: 52f38a2055ed ("app/testpmd: enable TCP/IPv4 VxLAN and GRE GSO")
Cc: stable@dpdk.org
Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>
Tested-by: Wei Ling <weix.ling@intel.com>
Acked-by: Yuying Zhang <yuying.zhang@intel.com>