lib/gro: add Generic Receive Offload API framework

lib/gro: add Generic Receive Offload API framework

Generic Receive Offload (GRO) is a widely used SW-based offloading
technique to reduce per-packet processing overhead. It gains
performance by reassembling small packets into large ones. This
patchset is to support GRO in DPDK. To support GRO, this patch
implements a GRO API framework.

To enable more flexibility to applications, DPDK GRO is implemented as
a user library. Applications explicitly use the GRO library to merge
small packets into large ones. DPDK GRO provides two reassembly modes.
One is called lightweight mode, the other is called heavyweight mode.
If applications want to merge packets in a simple way and the number
of packets is relatively small, they can use the lightweight mode.
If applications need more fine-grained controls, they can choose the
heavyweight mode.

rte_gro_reassemble_burst is the main reassembly API which is used in
lightweight mode and processes N packets at a time. For applications,
performing GRO in lightweight mode is simple. They just need to invoke
rte_gro_reassemble_burst. Applications can get GROed packets as soon as
rte_gro_reassemble_burst returns.

rte_gro_reassemble is the main reassembly API which is used in
heavyweight mode and tries to merge N inputted packets with the packets
in GRO reassembly tables. For applications, performing GRO in heavyweight
mode is relatively complicated. Before performing GRO, applications need
to create a GRO context object, which keeps reassembly tables of
desired GRO types, by rte_gro_ctx_create. Then applications can use
rte_gro_reassemble to merge packets. The GROed packets are in the
reassembly tables of the GRO context object. If applications want to get
them, applications need to manually flush them by flush API.

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>