libblkid: add support for UBI superblock

libblkid: add support for UBI superblock

UBI is a volume management system that can be used on a raw flash
partition for providing multiple logical volumes. Detecting UBI
superblock may be useful for tools wanting to simplify or automate
attaching UBI.

Please note it's not directly related to the ubifs support which is just
a filesystem working on top of UBI volume.

In other words: UBI can be used on MTD partition (e.g. /dev/mtdblock0)
while ubifs can be used on UBI volume (e.g. /dev/ubi0_0).

This patch adds simple code reading UBI version and unique number and
setting it in the blkid_probe.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>