x86/xsaves: Clear reserved bits in xsave header

The reserved bits (128~511) in the xsave header must be zero according to
X86 SDM. Clear the bits in this patch.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-12-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>