lib/fs: Use os.FileMode.String for fs.FileMode (#5302)

lib/fs/filesystem_test.go

@@ -98,3 +98,11 @@ func TestCanonicalize(t *testing.T) {
 		}
 	}
 }
+
+func TestFileModeString(t *testing.T) {
+	var fm FileMode = 0777
+	exp := "-rwxrwxrwx"
+	if fm.String() != exp {
+		t.Fatalf("Got %v, expected %v", fm.String(), exp)
+	}
+}