FIX: Last checkpoint is not initialized on reshape restart

FIX: Last checkpoint is not initialized on reshape restart

When reshape is restarted and active array in mdmon is being initialized,
mdmon has to know last checkpoint, otherwise reshape will be restarted
form '0' position.
mdadm when reshaped array is assembled stores reshape_position in sysfs
and runs mdmon. Initialize last_checkpoint in active array structure
to value present in sysfs for reshaped array start.

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