mm: kmsan: add tests for high-order page freeing

mm: kmsan: add tests for high-order page freeing

Add regression tests to verify that KMSAN correctly poisons the full
memory range when freeing pages.

Specifically, verify that accessing the tail pages of a high-order
non-compound allocation triggers a use-after-free report.  This ensures
that the fix "mm: kmsan: Fix poisoning of high-order non-compound pages"
is working as expected.

Also add a test for standard order-0 pages for completeness.

Link: https://lore.kernel.org/all/20260104134348.3544298-1-ryan.roberts@arm.com/
Link: https://lkml.kernel.org/r/20260113091151.4035013-1-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>