DO NOT MERGE THIS ONE!!

This commit drops RLC UM packets if the buffer occupancy is higher
than 16MB.

Let's say your UE can go up to 16Mb/s. If you do UDP iperf with
let's say 40Mb/s very quickly the buffer occupancy will reach
the 16Mb limit and following packets will get dropped.

Then something goes wrong. I don't know what. Just that iperf
reports very low throughput (I've seen down to 11 kb/s).
Stopping iperf, waiting for buffer to be empty and starting
again at let's say 10Mb/s does not provide 10Mb/s immediately,
but eventually it does, as if the RLC UM "goes back to normal".

So I don't know if it's a correct solution.

conclusion: DO NOT MERGE!!
Let's say it's work in progress, open for comments and
suggestions.

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

@@ -687,6 +687,14 @@ rlc_um_data_req (const protocol_ctxt_t* const ctxt_pP, void *rlc_pP, mem_block_t
   char                 message_string[7000];
 #endif
 
+  /* put a hard limit of 16MB */
+  if (rlc_p->buffer_occupancy > 16 * 1024 * 1024) {
+    free_mem_block(sdu_pP);
+    LOG_E(RLC, PROTOCOL_RLC_UM_CTXT_FMT" buffer full, dropping incoming data\n",
+          PROTOCOL_RLC_UM_CTXT_ARGS(ctxt_pP,rlc_p));
+    return;
+  }
+
   LOG_D(RLC, PROTOCOL_RLC_UM_CTXT_FMT" RLC_UM_DATA_REQ size %d Bytes, BO %d , NB SDU %d\n",
         PROTOCOL_RLC_UM_CTXT_ARGS(ctxt_pP,rlc_p),
         ((struct rlc_um_data_req *) (sdu_pP->data))->data_size,