gso: add Generic Segmentation Offload API framework

gso: add Generic Segmentation Offload API framework

Generic Segmentation Offload (GSO) is a SW technique to split large
packets into small ones. Akin to TSO, GSO enables applications to
operate on large packets, thus reducing per-packet processing overhead.

To enable more flexibility to applications, DPDK GSO is implemented
as a standalone library. Applications explicitly use the GSO library
to segment packets. To segment a packet requires two steps. The first
is to set proper flags to mbuf->ol_flags, where the flags are the same
as that of TSO. The second is to call the segmentation API,
rte_gso_segment(). This patch introduces the GSO API framework to DPDK.

rte_gso_segment() splits an input packet into small ones in each
invocation. The GSO library refers to these small packets generated
by rte_gso_segment() as GSO segments. Each of the newly-created GSO
segments is organized as a two-segment MBUF, where the first segment is a
standard MBUF, which stores a copy of packet header, and the second is an
indirect MBUF which points to a section of data in the input packet.
rte_gso_segment() reduces the refcnt of the input packet by 1. Therefore,
when all GSO segments are freed, the input packet is freed automatically.
Additionally, since each GSO segment has multiple MBUFs (i.e. 2 MBUFs),
the driver of the interface which the GSO segments are sent to should
support to transmit multi-segment packets.

The GSO framework clears the PKT_TX_TCP_SEG flag for both the input
packet, and all produced GSO segments in the event of success, since
segmentation in hardware is no longer required at that point.

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>