UBI: fastmap: fix backward compatibility with image_seq

Some old UBI implementations (e.g. U-Boot) have not implemented the image
sequence feature.
So, when erase blocks are written, the image sequence in the ec header
is lost (set to zero).
UBI scan_all() takes this case into account (commits
32bc4820 and
2eadaad6)

But fastmap scan functions (ubi_scan_fastmap() and scan_pool()) didn't.

This patch fixes the issue.

Signed-off-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>