mm/mempolicy: use PAGE_ALIGN instead of open-coding it

Replace the simple calculation with PAGE_ALIGN.

Link: https://lkml.kernel.org/r/20220913015505.1998958-1-zuoze1@huawei.com


Signed-off-by: ze zuo <zuoze1@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>