gtpu: support separate rx-decap and encap-tx teid values

gtpu: support separate rx-decap and encap-tx teid values

Support separate local and remote TEIDs, with local (or RX) one used
for GTPU tunnel RX/decap and remote (or TX) one used encap/TX.
Updated current gtpu API to support seperate RX/TX TEIDs and added new
gtpu_tunnel_update_tteid API to allow changing TX-TEID of an existing
GTPU tunnel.  The current tunnel field "teid" is used for RX-TEID and
a new field "tteid" is used for TX-TEID.

Type: improvement

Signed-off-by: John Lo <loj@cisco.com>
Change-Id: I549d79750a34bb965036da298c0ca894d15c2c20