ubi: fastmap: Correctly handle interrupted erasures in EBA

Fastmap cannot track the LEB unmap operation, therefore it can
happen that after an interrupted erasure the mapping still looks
good from Fastmap's point of view, while reading from the PEB will
cause an ECC error and confuses the upper layer.

Instead of teaching users of UBI how to deal with that, we read back
the VID header and check for errors. If the PEB is empty or shows ECC
errors we fixup the mapping and schedule the PEB for erasure.

Fixes: dbb7d2a8 ("UBI: Add fastmap core")
Cc: <stable@vger.kernel.org>
Reported-by: martin bayern <Martinbayern@outlook.com>
Signed-off-by: Richard Weinberger <richard@nod.at>