plugin/file: optimize cname get rr by type (#5080)

Signed-off-by: xuweiwei <xuweiwei_yewu@cmss.chinamobile.com>

plugin/file/lookup.go

@@ -366,7 +366,7 @@ Redo:
 		goto Redo
 	}
 
-	targets := rrutil.CNAMEForType(elem.All(), qtype)
+	targets := elem.Type(qtype)
 	if len(targets) > 0 {
 		rrs = append(rrs, targets...)
 

plugin/file/rrutil/util.go

@@ -16,14 +16,3 @@ func SubTypeSignature(rrs []dns.RR, subtype uint16) []dns.RR {
 	}
 	return sigs
 }
-
-// CNAMEForType returns the RR that have the qtype from targets.
-func CNAMEForType(rrs []dns.RR, qtype uint16) []dns.RR {
-	ret := []dns.RR{}
-	for _, target := range rrs {
-		if target.Header().Rrtype == qtype {
-			ret = append(ret, target)
-		}
-	}
-	return ret
-}