selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"

Patch series "selftests/mm: reuse FORCE_READ to replace "asm volatile("" :
"+r" (XXX));" and some cleanup", v2.

This series introduces a common FORCE_READ() macro to replace the cryptic
asm volatile("" : "+r" (variable)); construct used in several mm
selftests.  This improves code readability and maintainability by removing
duplicated, hard-to-understand code.


This patch (of 2):

Several mm selftests use the `asm volatile("" : "+r" (variable));`
construct to force a read of a variable, preventing the compiler from
optimizing away the memory access.  This idiom is cryptic and duplicated
across multiple test files.

Following a suggestion from David[1], this patch refactors this common
pattern into a FORCE_READ() macro

Link: https://lkml.kernel.org/r/20250717131857.59909-1-lianux.mm@gmail.com
Link: https://lkml.kernel.org/r/20250717131857.59909-2-lianux.mm@gmail.com
Link: https://lore.kernel.org/lkml/4a3e0759-caa1-4cfa-bc3f-402593f1eee3@redhat.com/

 [1]
Signed-off-by: wang lian <lianux.mm@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>