selftests: mm: restore settings from only parent process

The atexit() is called from parent process as well as forked processes. 
Hence the child restores the settings at exit while the parent is still
executing.  Fix this by checking pid of atexit() calling process and only
restore THP number from parent process.

Link: https://lkml.kernel.org/r/20240314094045.157149-1-usama.anjum@collabora.com
Fixes: c23ea617

 ("selftests/mm: protection_keys: save/restore nr_hugepages settings")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Joey Gouly <joey.gouly@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>