fix tunmap: mixup of Access/Core side FAR rules

For tunmap, forward to the correct side: when collecting PDR and FAR on
one side, we need to take its PDR and the *reverse* FAR, instead of the
FAR paired with that PDR:

  Access --PDR-1->|--FAR-1-> Core
         <-FAR-2--|<-PDR-2--

Related: SYS#6192
Change-Id: I66babdfe4c1746bd3bf259342ce80dae2661de8c
diff --git a/src/osmo-upf/up_session.c b/src/osmo-upf/up_session.c
index d2a405d..fe3d359 100644
--- a/src/osmo-upf/up_session.c
+++ b/src/osmo-upf/up_session.c
@@ -1324,13 +1324,13 @@
 		.tunmap = {
 			.access = {
 				.local_teid = pdr->local_f_teid->fixed.teid,
-				.remote_teid = far_forw->outer_header_creation.teid,
-				.gtp_remote_addr = far_forw->outer_header_creation.ip_addr.v4,
+				.remote_teid = rfar_forw->outer_header_creation.teid,
+				.gtp_remote_addr = rfar_forw->outer_header_creation.ip_addr.v4,
 			},
 			.core = {
 				.local_teid = rpdr->local_f_teid->fixed.teid,
-				.remote_teid = rfar_forw->outer_header_creation.teid,
-				.gtp_remote_addr = rfar_forw->outer_header_creation.ip_addr.v4,
+				.remote_teid = far_forw->outer_header_creation.teid,
+				.gtp_remote_addr = far_forw->outer_header_creation.ip_addr.v4,
 			},
 		},
 	};