volume_id: fix ntfs attribute search

len is declared as uint32_t but le16_to_cpu macro is used

Signed-off-by: Oliver Metz <oliver.metz@gmx.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Change-Id: Ibc342fcf970d98003fac15c7008b87622e38647f