[mount] [regression] `filer.path` option results in empty mount in 2.32

fix https://github.com/chrislusf/seaweedfs/issues/1906

weed/filesys/dir.go

@@ -552,5 +552,8 @@ func (dir *Dir) saveEntry() error {
 }
 
 func (dir *Dir) FullPath() string {
+	if dir.wfs.option.FilerMountRootPath == "/" {
 		return string(dir.entry.FullPath)
 	}
+	return string(dir.entry.FullPath)[len(dir.wfs.option.FilerMountRootPath):]
+}