selftests/mm: put general ksm operation into vm_util

selftests/mm: put general ksm operation into vm_util

Patch series "test that rmap behaves as expected", v4.

As David suggested, currently we don't have a high level test case to
verify the behavior of rmap. This patch set introduce the verification
on rmap by migration.

Patch 1 is a preparation to move ksm related operations into vm_util.
Patch 2 is the new test case for rmap.

Currently it covers following four scenarios:

  * anonymous page
  * shmem page
  * pagecache page
  * ksm page

This patch (of 2):

There are some general ksm operations could be used by other related
test cases. Put them into vm_util for common use.

This is a preparation patch for later use.

Link: https://lkml.kernel.org/r/20250819080047.10063-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20250819080047.10063-2-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>