imsm: Process reshape_update in mdmon

imsm: Process reshape_update in mdmon

For this update prepare_update() allocates memory to relink imsm
(bigger) device imsm structures. It calculates new /bigger/ anchor
size.

Process update applies update in to imsm structures.  This includes
 - converting selected spares into configured devices
 - marking the arrays as migrating
 - making a new 'map' for each array with the changed details.

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