RTC: Ratelimit "lost interrupts" message

We gets lots of these when the kernel is running on a hypervisor.  Zach says
"a guest kernel trying to get high frequency RTC will also be inaccurate, and
inevitably will have unhidable interrupt lateness."
Signed-off-by: Ben Collins <bcollins@ubuntu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/rtc.c

@@ -1159,6 +1159,7 @@ static void rtc_dropped_irq(unsigned long data)
 
 	spin_unlock_irq(&rtc_lock);
 
+	if (printk_ratelimit())
 		printk(KERN_WARNING "rtc: lost some interrupts at %ldHz.\n", freq);
 
 	/* Now we have new data */