Fixed #13174 -- Fixed missing field label for `readonly_fields` when used in...

Fixed #13174 -- Fixed missing field label for `readonly_fields` when used in `StackedInline`, thanks to benc for the report and ptone for the investigation and initial patch.
 * Corrected `InlineAdminForm.__init__` to pass along `model_admin` parameter in `super` call.
 * Lookup the field label in the form's model, not the `model_admin` model.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12857 bcc190cf-cafb-0310-a4f2-bffc1f526a37