Optimize Geneve option to be dynamic spaced

[Hong-Chang]

Current the Geneve options scaled_ep_opt, pod_label_value_opt and namesapce_label_value_opt will always take space in packet. Let's try whether there is a way to optimize the option space.

For example, for pod_label_value_opt and namesapce_label_value_opt, when there is no option needed, we can just ignore the setting of the options, and the options won't take any space in packet.

Need to change src/xdp/trn_agent_xdp.c (sender) and src/xdp/trn_transit_xdp.c (receiver) correspondingly for the optimization.