Editor: Check get_user_count() instead of get_users() for the locked post notice.

This aims to avoid slowing down the post editing by using a cached value instead of calling `get_users()`, which can be slow when plugins add various filters to all `get_users()` calls, especially with meta value comparisons on large `user_meta` tables.

Follow-up to [24304], [24543], [41829], [53011], [53018].

Props berislav.grgicak, bor0, costdev.
See #55958.

git-svn-id: https://develop.svn.wordpress.org/trunk@59522 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/edit-form-advanced.php

@@ -25,14 +25,7 @@
 if ( is_multisite() ) {
 	add_action( 'admin_footer', '_admin_notice_post_locked' );
 } else {
-	$check_users = get_users(
-		array(
-			'fields' => 'ID',
-			'number' => 2,
-		)
-	);
-
-	if ( count( $check_users ) > 1 ) {
+	if ( get_user_count() > 1 ) {
 		add_action( 'admin_footer', '_admin_notice_post_locked' );
 	}