CategoricalDtype construction: actually use fastpath (#17891)

Author: jorisvandenbossche

pandas/core/dtypes/dtypes.py

@@ -178,7 +178,7 @@ def _finalize(self, categories, ordered, fastpath=False):
         if categories is not None:
             categories = Index(categories, tupleize_cols=False)
             # validation
-            self._validate_categories(categories)
+            self._validate_categories(categories, fastpath=fastpath)
             self._validate_ordered(ordered)
         self._categories = categories
         self._ordered = ordered