s390/ap: Replace xchg() with WRITE_ONCE()

The result of xchg() is not used, and in addition it is used on a one byte
memory area which leads to inefficient code.

Use WRITE_ONCE() instead to achieve the same result with much less
generated code.

Acked-by: Harald Freudenberger <[email protected]>
Acked-by: Alexander Gordeev <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>

Author: hcahca

drivers/s390/crypto/ap_bus.c

@@ -453,7 +453,7 @@ static void ap_tasklet_fn(unsigned long dummy)
 	 * important that no requests on any AP get lost.
 	 */
 	if (ap_irq_flag)
-		xchg(ap_airq.lsi_ptr, 0);
+		WRITE_ONCE(*ap_airq.lsi_ptr, 0);
 
 	spin_lock_bh(&ap_queues_lock);
 	hash_for_each(ap_queues, bkt, aq, hnode) {