ceph: check folio PG_private bit instead of folio->private

The pages in the file mapping maybe reclaimed and reused by other
subsystems and the page->private maybe used as flags field or
something else, if later that pages are used by page caches again
the page->private maybe not cleared as expected.

Here will check the PG_private bit instead of the folio->private.

Cc: stable@vger.kernel.org
URL: https://tracker.ceph.com/issues/55421


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Luis Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>