fix(RLC): Count dropped DL SDUs by the RLC
When a downlink SDU was discarded by the RLC Acknowledged Mode entity
due to a full transmit buffer (tx_maxsize), the drop statistics were not
being updated. The txpdu_dd_pkts and txpdu_dd_bytes counters remained at
0. It should be noted that while the statistics are named txpdu_dd_*,
the drop at this stage occurs at the SDU level.
Robert Schmidt
8c224fde9e