powerpc/powernv: Fix fortify source warnings in opal-prd.c

As reported by Mahesh & Aneesh, opal_prd_msg_notifier() triggers a
FORTIFY_SOURCE warning:

memcpy: detected field-spanning write (size 32) of single field "&item->msg" at arch/powerpc/platforms/powernv/opal-prd.c:355 (size 4)
WARNING: CPU: 9 PID: 660 at arch/powerpc/platforms/powernv/opal-prd.c:355 opal_prd_msg_notifier+0x174/0x188 [opal_prd]
NIP opal_prd_msg_notifier+0x174/0x188 [opal_prd]
LR opal_prd_msg_notifier+0x170/0x188 [opal_prd]
Call Trace:
opal_prd_msg_notifier+0x170/0x188 [opal_prd] (unreliable)
notifier_call_chain+0xc0/0x1b0
atomic_notifier_call_chain+0x2c/0x40
opal_message_notify+0xf4/0x2c0

This happens because the copy is targeting item->msg, which is only 4
bytes in size, even though the enclosing item was allocated with extra
space following the msg.

To fix the warning define struct opal_prd_msg with a union of the header
and a flex array, and have the memcpy target the flex array.

Reported-by: "Aneesh Kumar K.V" <[e-mail]>
Reported-by: Mahesh Salgaonkar <[e-mail]>
Tested-by: Mahesh Salgaonkar <[e-mail]>
Reviewed-by: Mahesh Salgaonkar <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/platforms/powernv/opal-prd.c

···
 #include <linux/uaccess.h>
 
 
+struct opal_prd_msg {
+	union {
+		struct opal_prd_msg_header header;
+		DECLARE_FLEX_ARRAY(u8, data);
+	};
+};
+
 /*
  * The msg member must be at the end of the struct, as it's followed by the
  * message data.
  */
 struct opal_prd_msg_queue_item {
-	struct list_head		list;
-	struct opal_prd_msg_header	msg;
+	struct list_head	list;
+	struct opal_prd_msg	msg;
 };
 
 static struct device_node *prd_node;
···
 	int rc;
 
 	/* we need at least a header's worth of data */
-	if (count < sizeof(item->msg))
+	if (count < sizeof(item->msg.header))
 		return -EINVAL;
 
 	if (*ppos)
···
 			return -EINTR;
 	}
 
-	size = be16_to_cpu(item->msg.size);
+	size = be16_to_cpu(item->msg.header.size);
 	if (size > count) {
 		err = -EINVAL;
 		goto err_requeue;
···
 	if (!item)
 		return -ENOMEM;
 
-	memcpy(&item->msg, msg->params, msg_size);
+	memcpy(&item->msg.data, msg->params, msg_size);
 
 	spin_lock_irqsave(&opal_prd_msg_queue_lock, flags);
 	list_add_tail(&item->list, &opal_prd_msg_queue);