x86: fix freeze in x86_64 RTC update code in time_64.c

Fix hard freeze on x86_64 when the ntpd service calls 
update_persistent_clock()

A repeatable but randomly timed freeze has been happening in Fedora 6
and 7 for the last year, whenever I run the ntpd service on my AMD64x2
HP Pavilion dv9000z laptop.  This freeze is due to the use of
spin_lock(&rtc_lock) under the assumption (per a bad comment) that
set_rtc_mmss is called only with interrupts disabled.  The call from
ntp.c to update_persistent_clock is made with interrupts enabled.
Signed-off-by: NDavid P. Reed <dpreed@reed.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/time_64.c

@@ -82,18 +82,15 @@ static int set_rtc_mmss(unsigned long nowtime)
 	int retval = 0;
 	int real_seconds, real_minutes, cmos_minutes;
 	unsigned char control, freq_select;
+	unsigned long flags;
 
 /*
- * IRQs are disabled when we're called from the timer interrupt,
- * no need for spin_lock_irqsave()
+ * set_rtc_mmss is called when irqs are enabled, so disable irqs here
  */
-
-	spin_lock(&rtc_lock);
-
+	spin_lock_irqsave(&rtc_lock, flags);
 /*
  * Tell the clock it's being set and stop it.
  */
-
 	control = CMOS_READ(RTC_CONTROL);
 	CMOS_WRITE(control | RTC_SET, RTC_CONTROL);
 
@@ -138,7 +135,7 @@ static int set_rtc_mmss(unsigned long nowtime)
 	CMOS_WRITE(control, RTC_CONTROL);
 	CMOS_WRITE(freq_select, RTC_FREQ_SELECT);
 
-	spin_unlock(&rtc_lock);
+	spin_unlock_irqrestore(&rtc_lock, flags);
 
 	return retval;
 }