fuse: Don't print default if flags != 0

flagString(flags, "default") should return default only if flags == 0.

Noticed by Han-Wen: https://review.gerrithub.io/c/hanwen/go-fuse/+/555490/comment/9cbd50ed_d836c29c/

Without the fix added test fails as

    --- FAIL: TestFlagStringDefault (0.00s)
        print_test.go:48: flagString(8, "X") -> got "X,0x8" ;  want "0x8"

Change-Id: I5b9432741e30948d490b276c097fbcfe45ecd936

fuse/print.go

@@ -133,12 +133,12 @@ func flagString(names *flagNames, fl int64, def string) string {
 			}
 		}
 	}
-	if len(s) == 0 && def != "" {
-		s = []string{def}
-	}
 	if fl != 0 {
 		s = append(s, fmt.Sprintf("0x%x", fl))
 	}
+	if len(s) == 0 && def != "" {
+		return def
+	}
 
 	return strings.Join(s, ",")
 }

fuse/print_test.go

@@ -20,3 +20,33 @@ func TestFlagStringOrder(t *testing.T) {
 		}
 	}
 }
+
+// verify how flagString handles provided default.
+func TestFlagStringDefault(t *testing.T) {
+	names := newFlagNames(map[int64]string{
+		1: "AAA",
+		2: "BBB",
+		4: "CCC",
+	})
+
+	testv := []struct {
+		flags int64
+		def   string
+		strok string
+	}{
+		{0, "", ""},
+		{0, "X", "X"},
+		{1, "X", "AAA"},
+		{5, "X", "AAA,CCC"},
+		{8, "X", "0x8"},
+		{9, "X", "AAA,0x8"},
+	}
+
+	for _, test := range testv {
+		str := flagString(names, test.flags, test.def)
+		if str != test.strok {
+			t.Errorf("flagString(%x, %q) -> got %q ;  want %q",
+				test.flags, test.def, str, test.strok)
+		}
+	}
+}