FIX: delta_disk can have UnSet value

FIX: delta_disk can have UnSet value

Delta_disk can be set to UnSet value.
This can a cause to pass wrong parameter to reshape_super().
To avoid such situations raid_disks and delta_disks parameters
have to be passed to reshape_super() separately.
It will be up to reshape_super() function validation
and usage of this parameters to avoid not valid values.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>