UBI: introduce eraseblock counter variables

This is just a preparation patch which introduces several
'struct ubi_scan_info' fields which count eraseblocks of different
types. This will be used later on to decide whether it is safe to
format the flash or not. No functional changes so far.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Tested-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>