Upgrade/Install: Pass stored credentials to WP_Filesystem() in WP_Upgrader.

When using a non-direct filesystem, the call in `WP_Upgrader::maintenance_mode()` did not include the required credentials, leading to a fatal error as the connection was not initialized properly.

This commit attempts to use the stored credentials if available, and triggers a notice otherwise.

Follow-up to [56341], [58128].

Props hideishi, dd32, SergeyBiryukov.
Fixes #62718.

git-svn-id: https://develop.svn.wordpress.org/trunk@59981 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-upgrader.php

@@ -1007,9 +1007,17 @@ public function run( $options ) {
 	public function maintenance_mode( $enable = false ) {
 		global $wp_filesystem;
 
-		if ( ! $wp_filesystem ) {
-			require_once ABSPATH . 'wp-admin/includes/file.php';
-			WP_Filesystem();
+		if ( ! function_exists( 'WP_Filesystem' ) ) {
+			require_once ABSPATH . '/wp-admin/includes/file.php';
+		}
+
+		ob_start();
+		$credentials = request_filesystem_credentials( '' );
+		ob_end_clean();
+
+		if ( false === $credentials || ! WP_Filesystem( $credentials ) ) {
+			wp_trigger_error( __FUNCTION__, __( 'Could not access filesystem.' ) );
+			return;
 		}
 
 		$file = $wp_filesystem->abspath() . '.maintenance';