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.