Grow: add disks chosen by metadata handler to array for growth.

Grow: add disks chosen by metadata handler to array for growth.

With externally managed container based metadata, the ->reshape_super
method must choose any spares that are to be added to the array.
They should be prepared so that ->container_content will find them
as spares (disk.state == 0) which are assigned to a slot
(raid_disk >= 0).
We need to take those and add them to the array(s).

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