GTPv2/S2b: use correct instance id in the F-TEID

I've no idea why, but it seems the GTPu is the EBI, but the GTP-C is always 0.
This way it works with open5gs.

Change-Id: I053fe5661a38ce6fa7995964a0346129b56c823e
---
M src/epdg_gtpc_s2b.erl
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/epdg_gtpc_s2b.erl b/src/epdg_gtpc_s2b.erl
index bc1a076..0b76b6f 100644
--- a/src/epdg_gtpc_s2b.erl
+++ b/src/epdg_gtpc_s2b.erl
@@ -398,7 +398,7 @@
                   },
                   #v2_eps_bearer_id{eps_bearer_id = Bearer#gtp_bearer.ebi},
                   #v2_fully_qualified_tunnel_endpoint_identifier{
-                    instance = 0,
+                    instance = Bearer#gtp_bearer.ebi,
                     interface_type = 31, %% "S2b-U ePDG GTP-U"
                     key = Bearer#gtp_bearer.local_data_tei,
                     ipv4 = gtp_utils:ip_to_bin(LocalAddr)
@@ -408,7 +408,7 @@
            #v2_international_mobile_subscriber_identity{imsi = Imsi},
            #v2_rat_type{rat_type = 3}, %% 3 = WLAN
            #v2_fully_qualified_tunnel_endpoint_identifier{
-                instance = Bearer#gtp_bearer.ebi,
+                instance = 0,
                 interface_type = 30, %% "S2b ePDG GTP-C"
                 key = LocalCtlTEI,
                 ipv4 = gtp_utils:ip_to_bin(LocalAddr)
@@ -430,7 +430,7 @@
     Bearer = gtp_session_default_bearer(Sess),
     IEs = [#v2_eps_bearer_id{eps_bearer_id = Bearer#gtp_bearer.ebi},
            #v2_fully_qualified_tunnel_endpoint_identifier{
-               instance = Bearer#gtp_bearer.ebi,
+               instance = 0,
                interface_type = 30, %% "S2b ePDG GTP-C"
                key = Bearer#gtp_bearer.local_data_tei,
                ipv4 = gtp_utils:ip_to_bin(LocalAddr)

