Notify security keys change to CUUP during re-establishment

During re-establishment, new KgNB* is derived, requiring new UP keys (KUPint,
KUPenc) to be sent to CU-UP. When do_drb_integrity = true, CU-UP needs the
updated integrity keys to verify DRB PDUs. Without this update, CU-UP uses
old keys while the UE uses new keys, causing integrity failures.

The fix ensures security info is always sent during re-establishment,
which is critical for DRB integrity.

openair2/RRC/NR/rrc_gNB.c

@@ -1063,15 +1063,10 @@ static void cuup_notify_reestablishment(gNB_RRC_INST *rrc, gNB_RRC_UE_t *ue_p)
     }
   }
 
-#if 0
-  /* According to current understanding of E1 specifications, it is not needed
-   * to send security information because this does not change.
-   * But let's keep the code here in case it's needed.
-   */
-  // Always send security information
+  /* During reestablishment, for DRB integrity protection security keys change (KgNB* is derived),
+   * so security information MUST be sent to CU-UP to update DRB security keys. */
   req.secInfo = malloc_or_fail(sizeof(*req.secInfo));
   fill_security_info(rrc, ue_p, req.secInfo);
-#endif
 
   /* Send E1 Bearer Context Modification Request (3GPP TS 38.463) */
   sctp_assoc_t assoc_id = get_existing_cuup_for_ue(rrc, ue_p);