Add --data-offset flag for Create and Grow

Add --data-offset flag for Create and Grow

This can be used to over-ride the automatic assignment of
data offset.
For --create, it is useful to re-create old arrays where different
   defaults applied.
For --grow it may be able to force a reshape in the reverse direction.

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