SFF/SF Data Plane fixes

- SF decrements service index properly
- SFF checks if SI == 0
- If SI == 0 SFF decaps packet and transmits
- Properly handles packets with paths that do not exist
- Moved more variables to globals after testing
- Added VXLAN-GPE, NSH headers as comments in the code

Change-Id: I516dbbe36c061681f4f552e0a51e7a914b5715ae
Signed-off-by: Reinaldo Penno <rapenno@gmail.com>