mm/rodata_test: use PAGE_ALIGNED() helper

Use PAGE_ALIGNED() helper instead of open-coding operation, no functional
changes here.

Link: https://lkml.kernel.org/r/20220906075312.166595-1-xiujianfeng@huawei.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>