ubi: Deal with interrupted erasures in WL

When Fastmap is used we can face here an -EBADMSG
since Fastmap cannot know about unmaps.
If the erasure was interrupted the PEB may show ECC
errors and UBI would go to ro-mode as it assumes
that the PEB was check during attach time, which is
not the case with Fastmap.

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