libblkid: fix return codes from PART_ENTRY_* probing

libblkid: fix return codes from PART_ENTRY_* probing

The partitions prober does two step:

1) probe the assigned device for partition table

2) probe whole-disk device for partition table if the assigned device is a
   partition (this generates PART_ENTRY_* results for blkid -p -o udev))

The step 2) is optional and the return code from this probing should
not override success (rc=0) from the step 1) -- except situations when
the step 2) ends with I/O error or when the step 1) found nothing, but
2) was successful.

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