gso: support VxLAN GSO

gso: support VxLAN GSO

This patch adds a framework that allows GSO on tunneled packets.
Furthermore, it leverages that framework to provide GSO support for
VxLAN-encapsulated packets.

Supported VxLAN packets must have an outer IPv4 header (prepended by an
optional VLAN tag), and contain an inner TCP/IPv4 packet (with an optional
inner VLAN tag).

VxLAN GSO doesn't check if input packets have correct checksums and
doesn't update checksums for output packets. Additionally, it doesn't
process IP fragmented packets.

As with TCP/IPv4 GSO, VxLAN GSO uses a two-segment MBUF to organize each
output packet, which mandates support for multi-segment mbufs in the TX
functions of the NIC driver. Also, if a packet is GSOed, VxLAN GSO
reduces its MBUF refcnt by 1. As a result, when all of its GSO'd segments
are freed, the packet is freed automatically.

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