mm: kmsan: use helper macro offset_in_page()

Use helper macro offset_in_page() to improve code readability.  No
functional modification involved.

Link: https://lkml.kernel.org/r/20230727011612.2721843-3-zhangpeng362@huawei.com


Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Marco Elver <elver@google.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>