findmnt: allow to read (PART)UUID/LABEL from udev DB

Currently findmnt uses libmount to get filesystem properties (and
libmount uses libblkid). This works as expected, but root permissions
are required to read the tags from the devices.

It seems better to read the information from udev db and use libblkid
as fallback solution only.

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