mm/page_poison: remove unused page_ext.h from page_poison

Patch series "minor cleanups to page_ext header".

No page_ext function or structure is used in page_poison.  Just remove
page_ext header from page_poison.

Link: https://lkml.kernel.org/r/20230717113227.1897173-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20230717113227.1897173-2-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>