FIX: Fiddle raid_disks number when restarting reshape

FIX: Fiddle raid_disks number when restarting reshape

When restarting a reshape, the value of 'raid_disks' is the *new*
value.  The old value is found by subtracting delta_disks.
So before calling analyse_change we must set raid_disks to be the
old value, and then reset it afterwards.

All other fields are cleanly separated with the main field being
the 'old' value and a new_* field available.

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