support 'mc support perf object' with root login disabled (#19672)

It is expected that whoever is using the credentials which has
the proper set of permissions must be able to run.

`mc support perf object`

While the root login is disabled.

cmd/peer-rest-server.go

@@ -1059,6 +1059,13 @@ func (s *peerRESTServer) SpeedTestHandler(w http.ResponseWriter, r *http.Request
 	storageClass := r.Form.Get(peerRESTStorageClass)
 	bucketName := r.Form.Get(peerRESTBucket)
 	enableSha256 := r.Form.Get(peerRESTEnableSha256) == "true"
+	enableMultipart := r.Form.Get(peerRESTEnableMultipart) == "true"
+
+	u, ok := globalIAMSys.GetUser(r.Context(), r.Form.Get(peerRESTAccessKey))
+	if !ok {
+		s.writeErrorResponse(w, errAuthentication)
+		return
+	}
 
 	size, err := strconv.Atoi(sizeStr)
 	if err != nil {
@@ -1078,12 +1085,14 @@ func (s *peerRESTServer) SpeedTestHandler(w http.ResponseWriter, r *http.Request
 	done := keepHTTPResponseAlive(w)
 
 	result, err := selfSpeedTest(r.Context(), speedTestOpts{
-		objectSize:   size,
-		concurrency:  concurrent,
-		duration:     duration,
-		storageClass: storageClass,
-		bucketName:   bucketName,
-		enableSha256: enableSha256,
+		objectSize:      size,
+		concurrency:     concurrent,
+		duration:        duration,
+		storageClass:    storageClass,
+		bucketName:      bucketName,
+		enableSha256:    enableSha256,
+		enableMultipart: enableMultipart,
+		creds:           u.Credentials,
 	})
 	if err != nil {
 		result.Error = err.Error()