w1: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warnings:

drivers/w1/w1_netlink.c:198:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/w1/w1_netlink.c:219:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/Z_RflBe5iDGTMFjV@kspp
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250513105326.27385-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: GustavoARSilva

drivers/w1/w1_netlink.c

@@ -194,16 +194,16 @@ static void w1_netlink_queue_status(struct w1_cb_block *block,
 static void w1_netlink_send_error(struct cn_msg *cn, struct w1_netlink_msg *msg,
 	int portid, int error)
 {
-	struct {
-		struct cn_msg cn;
-		struct w1_netlink_msg msg;
-	} packet;
-	memcpy(&packet.cn, cn, sizeof(packet.cn));
-	memcpy(&packet.msg, msg, sizeof(packet.msg));
-	packet.cn.len = sizeof(packet.msg);
-	packet.msg.len = 0;
-	packet.msg.status = (u8)-error;
-	cn_netlink_send(&packet.cn, portid, 0, GFP_KERNEL);
+	DEFINE_RAW_FLEX(struct cn_msg, packet, data,
+			sizeof(struct w1_netlink_msg));
+	struct w1_netlink_msg *pkt_msg = (struct w1_netlink_msg *)packet->data;
+
+	*packet = *cn;
+	*pkt_msg = *msg;
+	packet->len = sizeof(*pkt_msg);
+	pkt_msg->len = 0;
+	pkt_msg->status = (u8)-error;
+	cn_netlink_send(packet, portid, 0, GFP_KERNEL);
 }
 
 /**
@@ -215,22 +215,20 @@ static void w1_netlink_send_error(struct cn_msg *cn, struct w1_netlink_msg *msg,
  */
 void w1_netlink_send(struct w1_master *dev, struct w1_netlink_msg *msg)
 {
-	struct {
-		struct cn_msg cn;
-		struct w1_netlink_msg msg;
-	} packet;
-	memset(&packet, 0, sizeof(packet));
+	DEFINE_RAW_FLEX(struct cn_msg, packet, data,
+			sizeof(struct w1_netlink_msg));
+	struct w1_netlink_msg *pkt_msg = (struct w1_netlink_msg *)packet->data;
 
-	packet.cn.id.idx = CN_W1_IDX;
-	packet.cn.id.val = CN_W1_VAL;
+	packet->id.idx = CN_W1_IDX;
+	packet->id.val = CN_W1_VAL;
 
-	packet.cn.seq = dev->seq++;
-	packet.cn.len = sizeof(*msg);
+	packet->seq = dev->seq++;
+	packet->len = sizeof(*msg);
 
-	memcpy(&packet.msg, msg, sizeof(*msg));
-	packet.msg.len = 0;
+	*pkt_msg = *msg;
+	pkt_msg->len = 0;
 
-	cn_netlink_send(&packet.cn, 0, 0, GFP_KERNEL);
+	cn_netlink_send(packet, 0, 0, GFP_KERNEL);
 }
 
 static void w1_send_slave(struct w1_master *dev, u64 rn)