x86_64: fix optimised var read in pit_wait()

arch/x86_64/pit.c

@@ -8,7 +8,6 @@ static unsigned long long tick_counter = 0;
 static int pit_callback(void)
 {
 	tick_counter++;
-	printk("tick");
 	return 0;
 }
 
@@ -18,6 +17,8 @@ static irq_hook_t pit_irq_hook = {
 
 void pit_start(unsigned int hz)
 {
+	tick_counter = 0;
+
 	unsigned int divisor = 1193180 / hz;
 
 	outportb(0x43, 0x36);
@@ -49,7 +50,6 @@ void pit_wait(unsigned int ticks)
 {
 	unsigned long long end = tick_counter + ticks;
 
-	while (tick_counter < end) {
-		ml_cpu_pause();
+	while (READ_ONCE(tick_counter) < end) {
 	}
 }