This is in 3GPP TS 38.425.
Because it was not done, there was a problem of interoperability
between openair CU and srsran DU. No DL traffic was reaching the UE.
A new GTPU API function is introduced: gtpv1uSendDirectWithNRUSeqNum().
It is only used in the gNB CU, in PDCP. So it was decided to not add a
new parameter to gtpv1uSendDirect() but instead add a new API function.