commit 23654188 upstream.

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.

Fixes: dbb7d2a8 ("UBI: Add fastmap core")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>