hotfix: fix bad RLC UM packet creation

Running TCP DL traffic with one connected UE showed a lot of
fluctuations in throughput. After analysis it was found that
sometimes the RLC UM PDU was not correct. It contained one byte
more than it should. On the receiver side, the TCP packet
contained in the RLC packet seems to be rejected by the TCP
stack of the UE (it has one byte more than it should),
leading to a brutal reduction of the throughput, probably due
to some congestion detection in the TCP implementation.
Or something.

This hotfix seems to solve the problem. Using iperf in downlink
with a 5MHz eNB, we see no more fluctuations, the traffic is
very steady at 16.8Mb/s, as reported by the iperf server running
on the phone. (17.5 in the PHY plot of the T tracer.)

A rewrite of both the MAC and RLC UM packet generation is needed.
The code is way too complex for what it does and may contain
several similar problems that only trigger in specific rare
conditions.

openair2/LAYER2/RLC/UM_v9.3.0/rlc_um_segment.c

@@ -172,7 +172,7 @@ rlc_um_segment_10 (const protocol_ctxt_t* const ctxt_pP, rlc_um_entity_t *rlc_pP
         test_pdu_remaining_size = 0;
         test_remaining_size_to_substract = 0;
         test_remaining_num_li_to_substract = 0;
-        pdu_remaining_size = pdu_remaining_size - (test_li_length_in_bytes ^ 3);
+        //pdu_remaining_size = pdu_remaining_size - (test_li_length_in_bytes ^ 3);
       } else if ((sdu_mngt_p->sdu_remaining_size + (test_li_length_in_bytes ^ 3)) < test_pdu_remaining_size ) {
         test_num_li += 1;
         num_fill_sdu += 1;
@@ -365,9 +365,9 @@ rlc_um_segment_10 (const protocol_ctxt_t* const ctxt_pP, rlc_um_entity_t *rlc_pP
               sdu_mngt_p->sdu_remaining_size,
               pdu_remaining_size - sdu_mngt_p->sdu_remaining_size);
 #endif
-#if !EXMIMO
-        assert(1!=1);
-#endif
+//#if !EXMIMO
+//        assert(1!=1);
+//#endif
         memcpy(data, data_sdu_p, sdu_mngt_p->sdu_remaining_size);
         // reduce the size of the PDU
         continue_fill_pdu_with_sdu = 0;