x86/xsaves: Add a kernel parameter noxsaves to disable xsaves/xrstors

x86/xsaves: Add a kernel parameter noxsaves to disable xsaves/xrstors

This patch adds a kernel parameter noxsaves to disable xsaves/xrstors feature.
The kernel will fall back to use xsaveopt and xrstor to save and restor
xstates. By using this parameter, xsave area occupies more memory because
standard form of xsave area in xsaveopt/xrstor occupies more memory than
compacted form of xsave area.

This patch adds a description of the kernel parameter noxsaveopt in doc.
The code to support the parameter noxsaveopt has been in the kernel before.
This patch just adds the description of this parameter in the doc.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-4-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>