cleanup: rapi v2 instance tags wrong attribute

This was changed in the past, but it seems this class was forgotten.

Reviewed-by: amishchenko

lib/rapi/rlib2.py

@@ -364,7 +364,7 @@ class R_2_instances_name_tags(baserlib.R_Generic):
 
     """
     return baserlib._Tags_PUT(constants.TAG_INSTANCE,
-                               self.post_data, name=self.items[0])
+                              self.req.request_post_data, name=self.items[0])
 
   def DELETE(self):
     """Delete a tag.