lsblk: add process_all_devices_inverse()

lsblk: add process_all_devices_inverse()

This is necessary to implement --inverse. Note that this new
implementation scans /sys/dev/block/ to get top-level devices
and than it calls process_one_device().

Note that standard non-inverse tree does not use process_one_device()
as it's more effective to scan /sys/block where are no partitions.

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