imsm: Add support for copy area and backup operations

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>