UBI: Call scan_all() with correct offset in error case

If we find an invalid fastmap we have to scan from the very beginning.
Otherwise we leak the first 64 PEBs.

Reported-and-tested-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>