lsblk: Display mountpoint even for top-level device

lsblk: Display mountpoint even for top-level device

If a filesystem is mounted on top-level block device
with existing partitions, the mountpoint is not displayed
in the lsblk output.

This situation can happen by a configuration mistake
and lsblk could be used to detect such a mistake.

This patch allows searching for a mountpoint for all displayed
devices, not only for leaf nodes.
(It should be pretty cheap operation, mtab is parsed only once.)

For example: lsblk /dev/loop1
NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
loop1       7:1    0  128M  0 loop /mnt/tst
└─loop1p1 259:0    0  127M  0 loop

Signed-off-by: Milan Broz <gmazyland@gmail.com>