[S390] cio: ensure single load of irq handler pointer

Add barrier to prevent compiler from reloading pointer to irq handler.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/airq.c

@@ -133,6 +133,8 @@ void do_adapter_IO(u8 isc)
 		while (word) {
 			if (word & INDICATOR_MASK) {
 				airq = airqs[isc][i];
+				/* Make sure gcc reads from airqs only once. */
+				barrier();
 				if (likely(airq))
 					airq->handler(&indicators[isc].byte[i],
 						      airq->drv_data);