plugin/file: guard against cname loops (#4387)

Automatically submitted.

core/dnsserver/server_tls.go

@@ -50,6 +50,7 @@ func (s *ServerTLS) Serve(l net.Listener) error {
 	// Only fill out the TCP server for this one.
 	s.server[tcp] = &dns.Server{Listener: l, Net: "tcp-tls", Handler: dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
 		ctx := context.WithValue(context.Background(), Key{}, s.Server)
+		ctx = context.WithValue(ctx, LoopKey{}, 0)
 		s.ServeDNS(ctx, w, r)
 	})}
 	s.m.Unlock()