gso: support UDP/IPv4 fragmentation

gso: support UDP/IPv4 fragmentation

This patch adds GSO support for UDP/IPv4 packets. Supported packets
may include a single VLAN tag. UDP/IPv4 GSO doesn't check if input
packets have correct checksums, and doesn't update checksums for
output packets (the responsibility for this lies with the application).
Additionally, UDP/IPv4 GSO doesn't process IP fragmented packets.

UDP/IPv4 GSO uses two chained MBUFs, one direct MBUF and one indrect
MBUF, to organize an output packet. The direct MBUF stores the packet
header, while the indirect mbuf simply points to a location within the
original packet's payload. Consequently, use of UDP GSO requires
multi-segment MBUF support in the TX functions of the NIC driver.

If a packet is GSO'd, UDP/IPv4 GSO reduces its MBUF refcnt by 1. As a
result, when all of its GSOed segments are freed, the packet is freed
automatically.

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Acked-by: Xiao Wang <xiao.w.wang@intel.com>