MAINTAINERS: add mm ksm section

MAINTAINERS: add mm ksm section

As part of the ongoing efforts to sub-divide memory management
maintainership and reviewership, establish a section for Kernel Samepage
Merging (KSM) and add appropriate maintainers and reviewers.

Link: https://lkml.kernel.org/r/20250515190404.203596-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: Xu Xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>