Changes

s1gw: make f_ConnHdlr_rx_session_modify_req() more flexible

Allow the caller to pass FAR ID, SEID, and E-RAB parameters (F-TEID),
so that it can also be used for testing the E-RAB MODIFY procedure.

Change-Id: I52317e79ac02cb4e321b49c2cd824de28e25a1c6