TS24.501
8.2.11 DL NAS transport
8.2.11.4 5GMM cause
The AMF shall include this IE when the Payload container IE
contains an uplink payload which was not forwarded and
the Payload container type IE is not set to "Multiple payloads".
-0-
As such, this function 'nas_5gs_send_gsm_reject()' must be used
only when an N1 SM message has been forwarded to the SMF.