selftests/ksm: ksm_functional_tests: add prctl unmerge test

selftests/ksm: ksm_functional_tests: add prctl unmerge test

Let's test whether setting PR_SET_MEMORY_MERGE to 0 after setting it to 1
will unmerge pages, similar to how setting MADV_UNMERGEABLE after setting
MADV_MERGEABLE would.

Link: https://lkml.kernel.org/r/20230422205420.30372-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Stefan Roesch <shr@devkernel.io>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>