mm/khugepaged: use list_xxx() helper to improve readability

mm/khugepaged: use list_xxx() helper to improve readability

In general, khugepaged_scan_mm_slot() iterates khugepaged_scan.mm_head list
to get a mm_struct for collapse memory.

Use list_xxx() helper would be more obvious to the list iteration
operation.

No functional change.

Link: https://lkml.kernel.org/r/20250822025732.9025-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>