MatrixFree: Only set up the inner faces when so requested

[kronbichler]

MatrixFree::reinit() would previously aggressively set up both interior faces and boundary faces also when only the boundary faces were requested (e.g., when using continuous elements with some Robin-type boundary conditions, one does not want to set up all interior faces which require more expensive data exchange routines). This is now fixed and the behavior is as documented:

dealii/include/deal.II/matrix_free/matrix_free.h

Lines 409 to 413 in 0516371

	* @note In order to be able to perform a `face_operation`
	* in the MatrixFree::loop(), this field must be set to a value different
	* from UpdateFlags::update_default.
	*/
	UpdateFlags mapping_update_flags_inner_faces;

This PR creates a backward compatibility problem, in the sense that some codes that relied on this (undocumented) feature will now not perform face integrals. To be done after we branched for the 9.5 release.