UBI: add truly corrupted PEBs to corrupted list

Start using the 'corr' list and add there PEBs which look truly corrupted,
which means they have corrupted VID header and the data which follows the
corrupted header does not contain all 0xFF bytes.

At the moment, this does not change UBI functionality much because these
PEBs will be erase when scanning finishes. But the plan is to teach UBI
preserving them.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>