mount: and libblkid: covert /dev/dm-N to /dev/mapper/<name>

mount: and libblkid: covert /dev/dm-N to /dev/mapper/<name>

 * mount(8) uses private device-mapper names in mtab

 * libblkid returns private device-mapper names when evaluate udev
   /dev/disk-by symlinks.

 * on systems where DM is fully integrated with udev the /dev/mapper/<name>
   files are symlinks to /dev/dm-N. It means we need a special care to hide
   private device-mapper names.

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