Makefile: fix format error reporting

"go fmt" always fixes code in place. As a result, running it again to
report formatting errors doesn't do anything.

The original intention probably was to check for errors and then
report them. This is what is done now, using gofmt directly and
it's diff output mode.

Makefile

@@ -39,11 +39,12 @@ push: container
 	docker push $(IMAGE_NAME):$(IMAGE_VERSION)
 
 test:
-	if ! [ $$(go fmt $$(go list ./... | grep -v vendor) | wc -l) -eq 0 ]; then \
-		echo "formatting errors:"; \
-		go fmt $$(go list ./... | grep -v vendor); \
-		false; \
-	fi
+	files=$$(find ./ -name '*.go' | grep -v '^./vendor' ); \
+        if [ $$(gofmt -d $$files | wc -l) -ne 0 ]; then \
+                echo "formatting errors:"; \
+                gofmt -d $$files; \
+                false; \
+        fi
 	go vet $$(go list ./... | grep -v vendor)
 	go test $$(go list ./... | grep -v vendor | grep -v "cmd/csi-sanity")
 	./hack/e2e.sh