Fix OC_Helper and change deprecated methods

lib/helper.php

@@ -760,7 +760,7 @@ class OC_Helper {
 		$post_max_size = OCP\Util::computerFileSize(ini_get('post_max_size'));
 		$maxUploadFilesize = min($upload_max_filesize, $post_max_size);
 
-		$freeSpace = OC_Filesystem::free_space($dir);
+		$freeSpace = \OC\Files\Filesystem::free_space($dir);
 		$freeSpace = max($freeSpace, 0);
 
 		return min($maxUploadFilesize, $freeSpace);
@@ -790,12 +790,12 @@ class OC_Helper {
 	 * Calculate the disc space
 	 */
 	public static function getStorageInfo() {
-		$rootInfo = OC_FileCache::get('');
+		$rootInfo = \OC\Files\Filesystem::getFileInfo('/');
 		$used = $rootInfo['size'];
 		if ($used < 0) {
 			$used = 0;
 		}
-		$free = OC_Filesystem::free_space();
+		$free = \OC\Files\Filesystem::free_space();
 		$total = $free + $used;
 		if ($total == 0) {
 			$total = 1; // prevent division by zero