s390: use WRITE_ONCE when re-allocating async stack

The code does:

S390_lowcore.async_stack = new + STACK_INIT_OFFSET;

But the compiler is free to first assign one value and
add the other value later. If a IRQ would be coming in
between these two operations, it would run with an invalid
stack. Prevent this by using WRITE_ONCE.
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -346,7 +346,7 @@ static int __init async_stack_realloc(void)
 	new = stack_alloc();
 	if (!new)
 		panic("Couldn't allocate async stack");
-	S390_lowcore.async_stack = new + STACK_INIT_OFFSET;
+	WRITE_ONCE(S390_lowcore.async_stack, new + STACK_INIT_OFFSET);
 	free_pages(old, THREAD_SIZE_ORDER);
 	return 0;
 }