Fix calculations for max_progress and completed.

Fix calculations for max_progress and completed.

'sync_completed' can sometimes have a value which is slightly high.
So round-down relevant values to new-chunk size and that is what we
want.

Subtract from component_size after scaling down rather than before as
that is easier.
Make sure max_progress never goes negative when reshaping backwards.

Signed-off-by: NeilBrown <neilb@suse.de>