selftests/mm: test that rmap behaves as expected

selftests/mm: test that rmap behaves as expected

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

The general idea is if migrate one shared page between processes, this
would be reflected in all related processes.  Otherwise, we have problem
in rmap.

Currently it covers following four scenarios:

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

Link: https://lkml.kernel.org/r/20250819080047.10063-3-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>