RTT smoothing: fix a rounding bug

We were currently rounding down the result, which could cause the
smoothed RTT to get permanently stuck below the RTT samples.

We now round either up or down, whichever gets us closer to the
current RTT sample.

message.c

@@ -597,7 +597,7 @@ parse_packet(const unsigned char *from, struct interface *ifp,
     /* We can calculate the RTT to this neighbour. */
     if(have_hello_rtt && hello_send_us && hello_rtt_receive_time) {
         int remote_waiting_us, local_waiting_us;
-        int rtt;
+        int rtt, smoothed_rtt;
         int old_rttcost, changed = 0;
         remote_waiting_us = neigh->hello_send_us - hello_rtt_receive_time;
         local_waiting_us = time_us(neigh->hello_rtt_receive_time) -
@@ -615,9 +615,11 @@ parse_packet(const unsigned char *from, struct interface *ifp,
         old_rttcost = neighbour_rttcost(neigh);
         if (valid_rtt(neigh)) {
             /* Running exponential average. */
-            neigh->rtt = (ifp->rtt_exponential_decay * MAX(rtt, 0)
-                          + (256 - ifp->rtt_exponential_decay) * neigh->rtt);
-            neigh->rtt /= 256;
+            smoothed_rtt = (ifp->rtt_exponential_decay * MAX(rtt, 0)
+                            + (256 - ifp->rtt_exponential_decay) * neigh->rtt);
+            /* Rounding (up or down) to get closer to the sample. */
+            neigh->rtt = (neigh->rtt >= rtt) ? smoothed_rtt / 256 :
+                (smoothed_rtt + 255) / 256;
         } else {
             /* We prefer to be conservative with new neighbours
                (higher RTT) */