union: fix mkdir at root with remote:/

Before the this fix if you specified remote:/ then the union backend
would fail to notice the root directory existed.

This was fixed by stripping the trailing / from the root.

See: https://forum.rclone.org/t/upgraded-from-1-45-to-1-54-now-cant-create-new-directory-within-union-mount/22284/

backend/union/upstream/upstream.go

@@ -67,7 +67,7 @@ func New(ctx context.Context, remote, root string, cacheTime time.Duration) (*Fs
 		return nil, err
 	}
 	f := &Fs{
-		RootPath:    root,
+		RootPath:    strings.TrimRight(root, "/"),
 		writable:    true,
 		creatable:   true,
 		cacheExpiry: time.Now().Unix(),