libblkid: remove optimization from verify( funrtion

Now libblkid (the cache based part) tries to probe for the cached
filesystem firstly. This optimization is broken, because:

  * new another superblock could be on the device and the original
    is already obsolete
  * we still need to probe for partitions and raids
  * the code was too fragile

The patch also suggests lsblk --fs in blkid.8 for end users. lsblk
read information from used db.

Reported-by: Andreas Hofmeister <andi@collax.com>
Signed-off-by: Karel Zak <kzak@redhat.com>