mm/khugepaged: make khugepaged_collapse_control static

mm/khugepaged: make khugepaged_collapse_control static

The global variable 'khugepaged_collapse_control' is not used outside of
mm/khugepaged.c.  Make it static to limit its scope.

Link: https://lkml.kernel.org/r/20260118192253.9263-14-shivankg@amd.com
Signed-off-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>