udf: Convert udf_readdir() to new directory iteration

[ Upstream commit 7cd7a36a ]

Convert udf_readdir() to new directory iteration functions.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>