roles/routeadv: don't use subnet length smaller than 64
dnsmasq will fail if subnet length is smaller than 64 since new re6st production network usually gives /48 to nodes, this fix is now required
/cc @tomo
dnsmasq will fail if subnet length is smaller than 64 since new re6st production network usually gives /48 to nodes, this fix is now required
/cc @tomo