libblkid: use correct block size for UDF probe

In UDF, Volume Structure Descriptors are always 2048 bytes long (ECMA-167,
3rd ed., §2/9.1), while filesystem sectors are the same size as the sectors
of the underlying media (e.g., UDF 2.01 §1.3.2). Before this commit, the
block size was estimated from VSD offsets, which gives incorrect answers for
non-optical media. Now, the UDF probe uses the device block size by default,
falling back to 2048-byte sectors for the case of optical-disk images.

Signed-off-by: Zachary Catlin <z@zc.is>