md/raid5: allow for change in data_offset while managing a reshape.

md/raid5: allow for change in data_offset while managing a reshape.

The important issue here is incorporating the different in data_offset
into calculations concerning when we might need to over-write data
that is still thought to be valid.

To this end we find the minimum offset difference across all devices
and add that where appropriate.

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