UBI: fix forward compatibility

Commit 0798cea8c2e1afee59686c51d27d0e96b05e42d1 "UBI: improve corrupted flash handling"
broke delet-compatible volumes handling - it introduced a limit of 8 eraseblocks which
may be corrupted. And delete-compatible eraseblocks are added to the "corrupted" list,
so if we'd have a large delete-compatible volume, UBI would refuse it.

The fix is to add delete-compatible volumes to the erase list instead. Indeed, they are
corrupted, we just have to erase them.

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