mm/page_ext: add common function to get client data from page_ext

mm/page_ext: add common function to get client data from page_ext

Patch series "add page_ext_data to get client data in page_ext".

Current clients get data from page_ext by adding offset which is auto
generated in page_ext core and exposes the data layout design inside
page_ext core.  This series adds a page_ext_data() to hide this from
clients.

Benefits include:

1. Future clients can call page_ext_data directly instead of defining
   a new function like get_page_owner to get the data.

2. There is no change to clients if the layout of page_ext data changes.

This patch (of 3):

Add common page_ext_data function to get client data.  This could hide
offset which is auto generated in page_ext core and expose the desgin of
page_ext data layout.

Link: https://lkml.kernel.org/r/20230718145812.1991717-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20230718145812.1991717-2-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>