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.

openair2/LAYER2/nr_rlc/nr_rlc_entity_am.c

@@ -1811,6 +1811,8 @@ void nr_rlc_entity_am_recv_sdu(nr_rlc_entity_t *_entity,
 
   if (entity->tx_size + size > entity->tx_maxsize) {
     entity->sdu_rejected++;
+    entity->common.stats.txpdu_dd_pkts++;
+    entity->common.stats.txpdu_dd_bytes += size;
     return;
   }