ceph: clear page dirty before invalidate page

clear_page_dirty_for_io(page) before mapping->a_ops->invalidatepage().
invalidatepage() clears page's private flag, if dirty flag is not
cleared, the page may cause BUG_ON failure in ceph_set_page_dirty().

Cc: stable@vger.kernel.org
Link: https://tracker.ceph.com/issues/40862


Signed-off-by: Erqi Chen <chenerqi@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>