mdadm/Grow: Update new level when starting reshape

mdadm/Grow: Update new level when starting reshape

Reshape needs to specify a backup file when it can't update data offset
of member disks. For this situation, first, it starts reshape and then
it kicks off mdadm-grow-continue service which does backup job and
monitors the reshape process. The service is a new process, so it needs
to read superblock from member disks to get information.

But in the first step, it doesn't update new level in superblock. So
it can't change level after reshape finishes, because the new level is
not right. So records the new level in the first step.

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>