memory tier: use helper macro __ATTR_RW()

Use helper macro __ATTR_RW to define numa demotion attributes.  Minor
readability improvement.

Link: https://lkml.kernel.org/r/20230715035111.2656784-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>