Don't increase seqno when receiving a request for an old seqno.
Don't increase seqno when receiving a request for an old seqno.

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