core/cpu: Call memset with proper cpu_thread offset

"cpu_thread *t + value" vs "(void *)t + val"

Fixes: cfe9d44 (core/cpu: Prevent clobbering of stack guard for boot-cpu)
CC: stable <skiboot@lists.ozlabs.org> # v6.0+
CC: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Acked-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Reviewed-by: Vaibhav Jain<vaibhav@linux.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

core/cpu.c

@@ -889,7 +889,7 @@ static void init_cpu_thread(struct cpu_thread *t,
 	const size_t guard_skip = container_off_var(t, stack_guard) +
 		sizeof(t->stack_guard);
 
-	memset(t + guard_skip, 0, sizeof(struct cpu_thread) - guard_skip);
+	memset(((void *)t) + guard_skip, 0, sizeof(struct cpu_thread) - guard_skip);
 	init_lock(&t->dctl_lock);
 	init_lock(&t->job_lock);
 	list_head_init(&t->job_queue);