selftests/mm:fix test_prctl_fork_exec return failure

selftests/mm:fix test_prctl_fork_exec return failure

After calling fork() in test_prctl_fork_exec(), the global variable
ksm_full_scans_fd is initialized to 0 in the child process upon entering
the main function of ./ksm_functional_tests.

In the function call chain test_child_ksm() -> __mmap_and_merge_range ->
ksm_merge-> ksm_get_full_scans, start_scans = ksm_get_full_scans() will
return an error.  Therefore, the value of ksm_full_scans_fd needs to be
initialized before calling test_child_ksm in the child process.

Link: https://lkml.kernel.org/r/20240617052934.5834-1-shechenglong001@gmail.com
Signed-off-by: aigourensheng <shechenglong001@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>