lsblk: fix processing of dm partition mappings

lsblk: fix processing of dm partition mappings

As blkid_devno_to_wholedisk returns parent dm device for
a partition mapping, the condition used in lsblk incorrectly
checked the parent-child relationship.

In this particular case, we need to process the dm partition
mapping like any other non-partition device as dm devices always
use proper holders/slaves sysfs hierarchy instead of
/sys/block/<parent>/<name> that is used for real partitions.

Example (test1 is a partition mapping and sdb1 is a real partition):
$ lsblk
NAME             MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda                8:0    0  128M  0 disk
`-test (dm-0)    253:0    0  128M  0 dm
  `-test1 (dm-1) 253:1    0  127M  0 part
sdb                8:16   0  128M  0 disk
`-sdb1             8:17   0  127M  0 disk

Before this patch (test1 skipped!):
$ lsblk -s /dev/mapper/test1
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
test (dm-0) 253:0    0  128M  0 dm
`-sda         8:0    0  128M  0 disk

$ lsblk -s /dev/sdb1
NAME  MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdb1    8:17   0  127M  0 disk
`-sdb   8:16   0  128M  0 disk

With this patch (test1 processed correctly):
$ lsblk -s /dev/mapper/test1
NAME          MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
test1 (dm-1)  253:1    0  127M  0 part
`-test (dm-0) 253:0    0  128M  0 dm
  `-sda         8:0    0  128M  0 disk

$ lsblk -s /dev/sdb1
NAME  MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdb1    8:17   0  127M  0 disk
`-sdb   8:16   0  128M  0 disk