Remove write_range from calculation of max_progress

Remove write_range from calculation of max_progress

It isn't needed as we always work in multiples of full
destination stripes.

Also multiply by 'after' disks, not before.

We can progress until the point we would write then lines up with
where we would read now.
We read now from
  array-address: reshape_progress   device-address: read_offset
So we write then to
  device-address: read_offset    array-address:  read_offset * after.disks

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