selftests/resctrl: Replace obsolete memalign() with posix_memalign()

memalign() is obsolete according to its manpage.

Replace memalign() with posix_memalign() and remove malloc.h include
that was there for memalign().

As a pointer is passed into posix_memalign(), initialize *p to NULL
to silence a warning about the function's return value being used as
uninitialized (which is not valid anyway because the error is properly
checked before p is returned).

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>