RTT smoothing: fix a rounding bug
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.

dtaht/babeld-shortrtt-metrics,jech/babeld,jech/babeld,tcatm/babeld,Drooids/babeld,woniullb/babeld,Gwendocg/babeldToS,Gwendocg/babeldToS,wlanslovenija/babeld,sudomesh/babeld,wlanslovenija/babeld,boutier/babeld