linkbox: fix upload error "user upload file not exist"

Linkbox have started issuing 302 redirects on some of their PUT
requests when rclone uploads a file.

This is problematic for several reasons:

1. This is the wrong redirect code - it should be 307 to preserve the method
2. Since Expect/100-Continue isn't supported the whole body gets uploaded

This fixes the problem by first doing a HEAD request on the URL. This
will allow us to read the redirect Location and not upload the body to
the wrong place.

It should still work (albeit a little more inefficiently) if Linkbox
stop redirecting the PUT requests.

See: https://forum.rclone.org/t/linkbox-upload-error/51795
Fixes: #8606

backend/linkbox/linkbox.go

@@ -617,16 +617,36 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	case 1:
 		// upload file using link from first step
 		var res *http.Response
+		var location string
+
+		// Check to see if we are being redirected
+		opts := &rest.Opts{
+			Method:     "HEAD",
+			RootURL:    getFirstStepResult.Data.SignURL,
+			Options:    options,
+			NoRedirect: true,
+		}
+		err = o.fs.pacer.CallNoRetry(func() (bool, error) {
+			res, err = o.fs.srv.Call(ctx, opts)
+			return o.fs.shouldRetry(ctx, res, err)
+		})
+		if res != nil {
+			location = res.Header.Get("Location")
+			if location != "" {
+				// set the URL to the new Location
+				opts.RootURL = location
+				err = nil
+			}
+		}
+		if err != nil {
+			return fmt.Errorf("head upload URL: %w", err)
+		}
 
 		file := io.MultiReader(bytes.NewReader(first10mBytes), in)
 
-		opts := &rest.Opts{
-			Method:        "PUT",
-			RootURL:       getFirstStepResult.Data.SignURL,
-			Options:       options,
-			Body:          file,
-			ContentLength: &size,
-		}
+		opts.Method = "PUT"
+		opts.Body = file
+		opts.ContentLength = &size
 
 		err = o.fs.pacer.CallNoRetry(func() (bool, error) {
 			res, err = o.fs.srv.Call(ctx, opts)