udf: Convert udf_unlink() to new directory iteration code

[ Upstream commit 6ec01a80 ]

Convert udf_unlink() to new directory iteration code.

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>