staging: vc04_services: Use scnprintf() for avoiding potential buffer overflow

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-rpi-kernel@lists.infradead.org
Cc: devel@driverdev.osuosl.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200319161300.25967-1-tiwai@suse.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c

@@ -2161,17 +2161,17 @@ int vchiq_dump_platform_service_state(void *dump_context,
 	char buf[80];
 	int len;
 
-	len = snprintf(buf, sizeof(buf), "  instance %pK", service->instance);
+	len = scnprintf(buf, sizeof(buf), "  instance %pK", service->instance);
 
 	if ((service->base.callback == service_callback) &&
 		user_service->is_vchi) {
-		len += snprintf(buf + len, sizeof(buf) - len,
+		len += scnprintf(buf + len, sizeof(buf) - len,
 			", %d/%d messages",
 			user_service->msg_insert - user_service->msg_remove,
 			MSG_QUEUE_SIZE);
 
 		if (user_service->dequeue_pending)
-			len += snprintf(buf + len, sizeof(buf) - len,
+			len += scnprintf(buf + len, sizeof(buf) - len,
 				" (dequeue pending)");
 	}