lsblk: split sysfs reading and scols table filling

lsblk: split sysfs reading and scols table filling

This change allows read devices from sysfs only once and reuse device
properties if the device is references more than once in the tree
(RAIDs, etc.).

* every device is in the tree only once (tree->devices list)

* iterate_block_devices() reuse already read devices (for example if
  already read for any dependence)

* the smartscols table is build from the final tree

The patch temporary disables dependencies evaluation (in
process_blkdev() to keep the patch small and simple.

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