Add link for more information
	279b706b x86,xen: introduce x86_init.mapping.pagetable_reserve

-v2: updated to commets from hpa to include commit name.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-29-git-send-email-yinghai@kernel.orgSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>