imsm: prepare update for level migrations reshape

imsm: prepare update for level migrations reshape

Introducing raid0->raid5 level migration metadata update structure
is prepared for future use.

Adding spare device is required to hold additional raid5 parity.
Mdadm just checks for spares, but it is not included in update.
If there are no spares available, abort. Otherwise we will create
degraded array what should be not allowed.
Mdmon will decide what spare device is used for parity.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: Przemyslaw Czarnowski <przemyslaw.hawrylewicz.czarnowski@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>