UBI: rename struct ubi_scan_info

Rename 'struct ubi_scan_info' to 'struct ubi_attach_info'. This is part
of the code re-structuring I am trying to do in order to add fastmap
in a more logical way. Fastmap can share a lot with scanning, including
the attach-time data structures, which all now have "scan" word in the
name. Let's get rid of this word.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>