afs: Use alternative invalidation to using launder_folio

Use writepages-based flushing invalidation instead of
invalidate_inode_pages2() and ->launder_folio().  This will allow
->launder_folio() to be removed eventually.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org