s390/kasan: avoid report in get_wchan

Reading other running task's stack can be a dangerous endeavor. Kasan
stack memory access instrumentation includes special prologue and epilogue
to mark/remove red zones in shadow memory between stack variables. For
that reason there is always a race between a task reading value in other
task's stack and that other task returning from a function and entering
another one generating different red zones pattern.

To avoid kasan reports simply perform uninstrumented memory reads.
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/process.c

@@ -196,12 +196,12 @@ unsigned long get_wchan(struct task_struct *p)
 		goto out;
 	}
 	for (count = 0; count < 16; count++) {
-		sf = (struct stack_frame *) sf->back_chain;
+		sf = (struct stack_frame *)READ_ONCE_NOCHECK(sf->back_chain);
 		if (sf <= low || sf > high) {
 			return_address = 0;
 			goto out;
 		}
-		return_address = sf->gprs[8];
+		return_address = READ_ONCE_NOCHECK(sf->gprs[8]);
 		if (!in_sched_functions(return_address))
 			goto out;
 	}