super1: simplify setting of array size.

super1: simplify setting of array size.

Currently the extra space to leave before the data in the array
is calculated in two separate places, and they can be inconsistent.

Instead, do it all in validate_geometry.  This records the
'data_offset' chosen which all other devices then use.

'write_init_super' now just uses the value rather than doing all the
calculations again.

This results in more consistent numbers.

Also, load_super sets st->data_offset so that it is used by "--add",
so the new device has a data offset matching a pre-existing device.

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