mm/ksm: test case for prctl fork/exec workflow

This adds a new test case to the ksm functional tests to make sure that
the KSM setting is inherited by the child process when doing a fork/exec.

Link: https://lkml.kernel.org/r/20230922211141.320789-3-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Carl Klemm <carl@uvos.xyz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>