lib/raid6: align AVX512 constants to 512 bits, not bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: mingo@redhat.com
Cc: Jim Kukunas <james.t.kukunas@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Megha Dey <megha.dey@linux.intel.com>
Cc: Gayatri Kammela <gayatri.kammela@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Shaohua Li <shli@fb.com>