drm: sysfs: 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().
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311073540.7108-1-tiwai@suse.de

drivers/gpu/drm/drm_sysfs.c

@@ -228,7 +228,7 @@ static ssize_t modes_show(struct device *device,
 
 	mutex_lock(&connector->dev->mode_config.mutex);
 	list_for_each_entry(mode, &connector->modes, head) {
-		written += snprintf(buf + written, PAGE_SIZE - written, "%s\n",
+		written += scnprintf(buf + written, PAGE_SIZE - written, "%s\n",
 				    mode->name);
 	}
 	mutex_unlock(&connector->dev->mode_config.mutex);