Fix QgsFeatureRequest with expression not using provider fields

and limit (fix #15771)

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -159,6 +159,8 @@ QgsVectorLayerFeatureIterator::QgsVectorLayerFeatureIterator( QgsVectorLayerFeat
       if ( source->mFields.fieldOrigin( idx ) != QgsFields::OriginProvider )
       {
         mProviderRequest.disableFilter();
+        // can't limit at provider side
+        mProviderRequest.setLimit( -1 );
       }
     }
   }