mdadm/Grow: Update reshape_progress to need_back after reshape finishes

mdadm/Grow: Update reshape_progress to need_back after reshape finishes

It tries to update data offset when kicking off reshape. If it can't
change data offset, it needs to use child_monitor to monitor reshape
progress and do back up job. And it needs to update reshape_progress
to need_back when reshape finishes. If not, it will be in a infinite
loop.

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