mm/page_ext: use page_ext_data helper in page_table_check

Use page_ext_data helper in page_table_check to avoid access offset
directly.

Link: https://lkml.kernel.org/r/20230718145812.1991717-3-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Andrew Morton <akpm@linux-foudation.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>