vet: Fix all the go vet complaints (#4029)

```
go tool vet -atomic -bool -copylocks -nilfunc \
   -printf -shadow -rangeloops -unreachable \
   -unsafeptr -unusedresult cmd/
```

Makefile

@@ -79,8 +79,8 @@ verifiers: vet fmt lint cyclo spelling
 
 vet:
 	@echo -n "Running $@: "
-	@go vet github.com/minio/minio/cmd/...
-	@go vet github.com/minio/minio/pkg/...
+	@go tool vet -atomic -bool -copylocks -nilfunc -printf -shadow -rangeloops -unreachable -unsafeptr -unusedresult cmd
+	@go tool vet -atomic -bool -copylocks -nilfunc -printf -shadow -rangeloops -unreachable -unsafeptr -unusedresult pkg
 	@echo "Done."
 
 fmt:

cmd/admin-handlers_test.go

@@ -1162,9 +1162,9 @@ func TestHealUploadHandler(t *testing.T) {
 	for i, test := range testCases {
 		// Prepare query params.
 		queryVal := mkHealUploadQuery(test.bucket, test.object, uploadID, test.dryrun)
-		req, err := buildAdminRequest(queryVal, "upload", http.MethodPost, 0, nil)
-		if err != nil {
-			t.Fatalf("Test %d - Failed to construct heal object request - %v", i+1, err)
+		req, err1 := buildAdminRequest(queryVal, "upload", http.MethodPost, 0, nil)
+		if err1 != nil {
+			t.Fatalf("Test %d - Failed to construct heal object request - %v", i+1, err1)
 		}
 		rec := httptest.NewRecorder()
 		adminTestBed.mux.ServeHTTP(rec, req)

cmd/azure-anonymous.go

@@ -60,8 +60,8 @@ func (a AzureObjects) AnonGetBucketInfo(bucket string) (bucketInfo BucketInfo, e
 // AnonGetObject - SendGET request without authentication.
 // This is needed when clients send GET requests on objects that can be downloaded without auth.
 func (a AzureObjects) AnonGetObject(bucket, object string, startOffset int64, length int64, writer io.Writer) (err error) {
-	url := a.client.GetBlobURL(bucket, object)
-	req, err := http.NewRequest("GET", url, nil)
+	u := a.client.GetBlobURL(bucket, object)
+	req, err := http.NewRequest("GET", u, nil)
 	if err != nil {
 		return azureToObjectError(traceError(err), bucket, object)
 	}

cmd/logger.go

@@ -153,20 +153,21 @@ func (log *Logger) Printf(format string, args ...interface{}) {
 	}
 }
 
-// NewLogger - returns new logger.
+// NewLogger - returns a new initialized logger.
 func NewLogger() *Logger {
 	logger := logrus.New()
 	logger.Out = ioutil.Discard
 	logger.Level = logrus.DebugLevel
 
-	log := &Logger{
+	l := &Logger{
 		logger:        logger,
 		consoleTarget: NewConsoleLogger(),
 	}
 
-	logger.Hooks.Add(log)
+	// Adds a console logger.
+	logger.Hooks.Add(l)
 
-	return log
+	return l
 }
 
 func getSource() string {

cmd/xl-v1-healing_test.go

@@ -534,10 +534,10 @@ func TestHealObjectXL(t *testing.T) {
 
 	var uploadedParts []completePart
 	for _, partID := range []int{2, 1} {
-		pInfo, err := obj.PutObjectPart(bucket, object, uploadID, partID,
+		pInfo, err1 := obj.PutObjectPart(bucket, object, uploadID, partID,
 			int64(len(data)), bytes.NewReader(data), "", "")
-		if err != nil {
-			t.Fatalf("Failed to upload a part - %v", err)
+		if err1 != nil {
+			t.Fatalf("Failed to upload a part - %v", err1)
 		}
 		uploadedParts = append(uploadedParts, completePart{
 			PartNumber: pInfo.PartNumber,