imsm: prepare memory for level migration update

imsm: prepare memory for level migration update

When level is changed from raid0 to raid5 memory is required for replace device
smaller device/array object.
This memory is allocated in manager context in prepare_update()

Prepare_update() is called in manager context so memory allocation are
allowed here. This allows us to look for spare devices for meta update.

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>