bdi/stable_pages_required attribute was deprecated in 5.10 and now
always returns 0. The replacement is queue/stable_writes. (It is
also writeable, so we can simplify these test cases somewhat in the
future.)
Fixes: https://tracker.ceph.com/issues/48232
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
