imsm: Add support for copy area and backup operations
This patch adds methods of manipulating migration record:
init_migr_record_imsm() - initiate migration record at the beginning of
the reshape process
write_imsm_migr_rec() - saves migration record to array.
Migration record is stored on 2 first disks in array only.
save_backup_imsm() - saves critical data stripes to Migration Copy Area
and updates the current migration unit status.
Uses restore_stripes() to format a destination stripe, and to write it
to the Migration Copy Area.
save_checkpoint_imsm() - Updates the current unit status in the
migration record.
Migration record is written to 2 first array disks only (similar to reading
operation).
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>