Avoid confusing with 'blocks' number.

Avoid confusing with 'blocks' number.

The 'blocks' number computed by analyse_change is the number of
blocks that it makes sense to back-up at a time.
It is the smallest number of blocks that is a whole number of
stripes in both the old and the new layout.

However we are also using it as the smallest amount of progress
that can be made at a time, which is wrong as it is always valid
to progress a single stripe in the new layout.

So change 'blocks' to be called 'backup_blocks' to make it more clear.
And pass new_chunk size down so it can be used for 'minimum forward
progress' calculations.

Also set 'stripes' (the amount actually backed up) from the
possibly-scaled 'blocks' number rather than ignoring it and using
backup_blocks.

Finally, get rid of 'read_range' as it isn't used (or needed).

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