Make udelay more accurate

From: Ross Biro <ross.biro@gmail.com>

The attempt to avoid overflow in __delay caused varying precision 
on different CPUs depending on differences in the CPU speed.

We should be able to do this multiplication with out overflowing
provided the
cpu is running at less than about 128 GHz.  xloops < 20000 * 0x10c6.
loops_per_jiffy * HZ <= cpu_clock_speed.  So if the cpu clock speed
< 2^64/(20000 * 0x10c6) = 2^64/ 51E6CC0 < 2^64/2^27 = 2^37 = 128G we
will not overflow the calculation.

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/lib/delay.c
===================================================================
--- linux.orig/arch/x86_64/lib/delay.c
+++ linux/arch/x86_64/lib/delay.c
@@ -39,7 +39,7 @@ void __delay(unsigned long loops)
 
 inline void __const_udelay(unsigned long xloops)
 {
-	__delay(((xloops * cpu_data[raw_smp_processor_id()].loops_per_jiffy) >> 32) * HZ);
+	__delay((xloops * HZ * cpu_data[raw_smp_processor_id()].loops_per_jiffy) >> 32);
 }
 
 void __udelay(unsigned long usecs)