x86: udelay: Use this_cpu_read to avoid address calculation

The code will use a segment prefix instead of doing the lookup and
calculation.

Signed-off-by: Christoph Lameter <cl@linux.com>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Tejun Heo <tj@kernel.org>