Micro-optimise mask_prefix.
Micro-optimise mask_prefix.

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