userspace: Add GTP-U support.

userspace: Add GTP-U support.

GTP, GPRS Tunneling Protocol, is a group of IP-based communications
protocols used to carry general packet radio service (GPRS) within
GSM, UMTS and LTE networks.  GTP protocol has two parts: Signalling
(GTP-Control, GTP-C) and User data (GTP-User, GTP-U). GTP-C is used
for setting up GTP-U protocol, which is an IP-in-UDP tunneling
protocol. Usually GTP is used in connecting between base station for
radio, Serving Gateway (S-GW), and PDN Gateway (P-GW).

This patch implements GTP-U protocol for userspace datapath,
supporting only required header fields and G-PDU message type.
See spec in:
https://tools.ietf.org/html/draft-hmm-dmm-5g-uplane-analysis-00

Tested-at: https://travis-ci.org/github/williamtu/ovs-travis/builds/666518784
Signed-off-by: Feng Yang <yangfengee04@gmail.com>
Co-authored-by: Feng Yang <yangfengee04@gmail.com>
Signed-off-by: Yi Yang <yangyi01@inspur.com>
Co-authored-by: Yi Yang <yangyi01@inspur.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Ben Pfaff <blp@ovn.org>