imsm: Prepare reshape_update in mdadm

imsm: Prepare reshape_update in mdadm

During Online Capacity Expansion metadata has to be updated to show
array changes and allow for future assembly of array.  To do this
mdadm prepares and sends reshape_update metadata update to mdmon.
The update contains the old and new number of raid disks, and the
indices of the spare disks that will be used to fill the spaces.

This works as follows:
1. reshape_super() prepares metadata update.
2. mdadm discovers the spares and adds them to the array
3. mdadm sends the update to mdmon
4. managemon in prepare_update() allocates required memory for bigger
   device object
5. monitor in process_update() updates the metadata to record the
   new sizes and the newly assigned devices.
6. mdadm initiates the reshape

Based on code From: Adam Kwolek <adam.kwolek@intel.com>

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