UBI: erase free PEB with bitflip in EC header

commit 193819cf2e6e395b1e1be2d36785dc5563a6edca upstream.

Without this patch, these PEB are not scrubbed until we put data in them.
Bitflip can accumulate latter and we can loose the EC header (but VID header
should be intact and allow to recover data).

Signed-off-by: Matthieu Castet <matthieu.castet@parrot.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>