Grow: allow for different sized devices when updating data_offset.

Grow: allow for different sized devices when updating data_offset.

It is possible that the devices in an array have different sizes, and
different data_offsets.  So the 'before_space' and 'after_space' may
be different from drive to drive.
Any decisions about how much to change the data_offset must work on
all devices, so must be based on the minimum available space on
any devices.

So find this minimum first, then do the calculation.

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