imsm: Add metadata update type for general migration check-pointing

imsm: Add metadata update type for general migration check-pointing

There are 2 places for keeping checkpoint information:
- metadata (per volume information used during volume
  initialization and rebuilding).
- migration record (per container information used during
  migration/reshape)

During reshape both checkpoints has to contains the same information.
To do this mdadm will send metadta update with checkpoint information.

Note: Checkpoint information consistence is not critical. During general
      migration restart, information from migration record is used only.

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