blkid: correct an erroneous error message

blkid: correct an erroneous error message

Running `blkid -p -n no /dev/sda2` would report:

  blkid: error: -u <list> argument is empty

(Mentioning -u instead of -n, and falsely claiming emptiness.)

Now the above command will report:

  blkid: invalid argument: no

(I don't think option -n needs to be mentioned, as it will be obvious
when inspecting the command line.  Not mentioning the option allows
reusing an existing error message, avoiding needless ballooning and
allowing the patch to be applied to the stable branch.)

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>