imsm: Add migration record to intel_super

imsm: Add migration record to intel_super

IMSM for securing reshape process uses special disk area outside metadata
for reshaped area backup purposes. If just reshaped array area requires
backup, bunch of array stripes prepared for reshape is stored in to
Migration Copy Area. In case of reshape interruption, Option ROM during
restart or mdadm during reshape restart (when no reboot occurs) will
restore Migration Copy Area to designation array. Reshape can be
continued from stable array stable state.

This patch adds support for IMSM migration record structure.
IMSM migration record is stored on the first two disks of IMSM volume
during the migration.

Add function for reading migration record, so mdadm can read (if present)
migration record. Migration record has to be cleared every time
MIGR_GEN_MIGR is started.

Signed-off-by: Maciej Trela <maciej.trela@intel.com>
Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: Krzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>