Multisite: Rename the calculate_current_dirsize filter to pre_recurse_dirsize.

Set the default value to `false`. This brings some consistency with the `pre_get_space_used` filter.

Follow-up to [49212], [49616], [49628].

See #19879.

git-svn-id: https://develop.svn.wordpress.org/trunk@49629 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-functions.php

@@ -2629,6 +2629,7 @@ function get_space_used() {
 	 * @param int|false $space_used The amount of used space, in megabytes. Default false.
 	 */
 	$space_used = apply_filters( 'pre_get_space_used', false );
+
 	if ( false === $space_used ) {
 		$upload_dir = wp_upload_dir();
 		$space_used = get_dirsize( $upload_dir['basedir'] ) / MB_IN_BYTES;