plugin/cache: don't update freq is prefetch isn't used. (#1144)
plugin/cache: don't update freq is prefetch isn't used. (#1144)

Small optimization.
coredns/coredns,jremond/coredns,chrisohaver/coredns,johnbelamaric/coredns,ii/coredns,ii/coredns,miekg/coredns,johnbelamaric/coredns,yongtang/coredns,jremond/coredns,miekg/coredns