fix: flow description should accept token "assigned"

[brianchennn]

Description

This PR fixed 2 Issues:

1. NULL SDF

• When SDF's flow description is set to permit out ip from any to assigned
• use sudo ./gtp5g-tunnel list pdr to observe, SDF is NULL
  

2. Wrong Source, Destination IP/Port

Since SMF created PDR which flow description should be same for both UL/DL, it's necessary to swap src, dst in UPF for UL before set rule into gtp5g.

Relative Spec

• TS 29.212 5.4.2
  
• TS 29.244 5.2.1A.2A