selftests/mm: add fork inheritance test for ksm_merging_pages counter

Add a new selftest to verify whether the `ksm_merging_pages` counter in
`mm_struct` is not inherited by a child process after fork.  This helps
ensure correctness of KSM accounting across process creation.

Link: https://lkml.kernel.org/r/e7bb17d374133bd31a3e423aa9e46e1122e74971.1758648700.git.donettom@linux.ibm.com


Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>