libblkid: populate jbd LOGUUID in addition to UUID

The LOGUUID is alias for UUID, this alias is necessary to standardize
the tag name (we want to use the same tag LOGUUID= for all external
logs on all filesystems).

Note that for another filesystems (e.g. xfs) we have to differentiate
between the journal and filesystem uuids to avoid collisions, because
both devices use the same uuid.

Signed-off-by: Karel Zak <kzak@redhat.com>