selftests/vm/pkeys: Use provided __cpuid_count() macro

kselftest.h makes the __cpuid_count() macro available
to conveniently call the CPUID instruction.

Remove the local CPUID wrapper and use __cpuid_count()
from already included kselftest.h instead.

__cpuid_count() from kselftest.h is used instead of the
macro provided by the compiler since gcc v4.4 (via cpuid.h)
because the selftest needs to be compiled with gcc v3.2,
the minimal required version for stable kernels.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: "Desnes A. Nunes do Rosario" <desnesn@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Suchanek <msuchanek@suse.de>
Cc: linux-mm@kvack.org
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>