selftest/mm: fix ksm_funtional_test failures

selftest/mm: fix ksm_funtional_test failures

This patch fixes 2 issues.

1) After fork() in test_prctl_fork, the child process uses the file
   descriptors from the parent process to read ksm_stat and
   ksm_merging_pages.  This results in incorrect values being read (parent
   process ksm_stat and ksm_merging_pages will be read in child), causing
   the test to fail.

   This patch calls init_global_file_handles() in the child process to
   ensure that the current process's file descriptors are used to read
   ksm_stat and ksm_merging_pages.

2) All tests currently call ksm_merge to trigger page merging.  To
   ensure the system remains in a consistent state for subsequent tests,
   it is better to call ksm_unmerge during the test cleanup phase

   In the test_prctl_fork test, after a fork(), reading
   ksm_merging_pages in the child process returns a non-zero value because
   a previous test performed a merge, and the child's memory state is
   inherited from the parent.

   Although the child process calls ksm_unmerge, the ksm_merging_pages
   counter in the parent is reset to zero, while the child's counter
   remains unchanged.  This discrepancy causes the test to fail.

   To avoid this issue, each test should call ksm_unmerge during
   cleanup to ensure the counter is reset and the system is in a clean
   state for subsequent tests.

execv argument is an array of pointers to null-terminated strings.  In
this patch we also added NULL in the execv argument.

Link: https://lkml.kernel.org/r/20250816040113.760010-4-aboorvad@linux.ibm.com
Fixes: 6c47de3be3a0 ("selftest/mm: ksm_functional_tests: extend test case for ksm fork/exec")
Co-developed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>