plugin/cache: don't update freq is prefetch isn't used.
Small optimization.
@@ -34,7 +34,9 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
resp, _ = state.Scrub(resp)
w.WriteMsg(resp)
+ if c.prefetch > 0 {
i.Freq.Update(c.duration, now)
+ }
pct := 100
if i.origTTL != 0 { // you'll never know
