imsm: Implement imsm_manage_reshape(), reshape workhorse

imsm: Implement imsm_manage_reshape(), reshape workhorse

Before reshape is started, mdadm should check again if there is only one
array (in container) under reshape. Then function "divides" array in to
"migration units" that can fits migration copy area and enters main loop.
It checks if current "migration unit" requires to be backed up.
If necessary mdadm saves it to copy area and updates migration record.
Then MD-driver is directed to perform reshape step (by "migration unit" size)
and checkpoint is moved forward. In this way reshape is executed until
array ends.

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>