blkid: use /dev/mapper/<name> rather than /dev/dm-<N>

blkid: use /dev/mapper/<name> rather than /dev/dm-<N>

The libblkid (since v1.41.1) returns private device-mapper names (e.g.
/dev/dm-0). It's because the probe_one() function scans /dev before
/dev/mapper.

brw-rw---- 1 root disk 253, 0 2009-04-27 13:41 /dev/dm-0
brw-rw---- 1 root disk 253, 0 2009-04-27 13:41 /dev/mapper/TestVolGroup-TestLogVolume

Old version:
  # blkid -t LABEL="TEST-LABEL" -o device
  /dev/dm-0

Fixed version:
  # blkid -t LABEL="TEST-LABEL" -o device
  /dev/mapper/TestVolGroup-TestLogVolume

Addresses-Red-Hat-Bug: #497259
Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>