fs: fix option types printing incorrectly for backend flags

Before this change backend types were printing incorrectly as the name
of the type, not what was defined by the Type() method.

This was not working due to not calling the Type() method. However
this needed to be defined on a non-pointer type due to the way the
options are handled.

fs/sizesuffix_test.go

@@ -17,8 +17,20 @@ type flagger interface {
 	json.Unmarshaler
 }
 
-// Check it satisfies the interface
-var _ flagger = (*SizeSuffix)(nil)
+// Interface which non-pointer flags must satisfy
+//
+// These are from pflag.Value and need to be non-pointer due the the
+// way the backend flags are inserted into the flags.
+type flaggerNP interface {
+	String() string
+	Type() string
+}
+
+// Check it satisfies the interfaces
+var (
+	_ flagger   = (*SizeSuffix)(nil)
+	_ flaggerNP = SizeSuffix(0)
+)
 
 func TestSizeSuffixString(t *testing.T) {
 	for _, test := range []struct {