lib/sysfs: fix sysfs_devname_to_devno for dm partitions

lib/sysfs: fix sysfs_devname_to_devno for dm partitions

Partitions mapped by device-mapper are not like real partitions where
there's a /sys/block/<parent>/<name>/dev sysfs path. We need to look
at /sys/block/<name>/dev like we do for any other non-partition devices.
The mapped partition is not found otherwise.

For example, this bug shows up in lsblk while specifying a device
on command line while that device is a dm mapping over a 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

Before this patch:
$lsblk /dev/mapper/test1
lsblk: dm-1: unknown device name

With this patch:
$lsblk /dev/mapper/test1
NAME         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
test1 (dm-1) 253:1    0  127M  0 part