um: Prevent KASAN splats in dump_stack()

Use READ_ONCE_NOCHECK() when reading the stack to prevent KASAN splats
when dump_stack() is used.

Fixes: 5b30140 ("UML: add support for KASAN under x86_64")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/sysrq.c

@@ -48,7 +48,8 @@ void show_stack(struct task_struct *task, unsigned long *stack,
 			break;
 		if (i && ((i % STACKSLOTS_PER_LINE) == 0))
 			pr_cont("\n");
-		pr_cont(" %08lx", *stack++);
+		pr_cont(" %08lx", READ_ONCE_NOCHECK(*stack));
+		stack++;
 	}
 
 	printk("%sCall Trace:\n", loglvl);