md-cluster/raid10: call update_size in md_reap_sync_thread

We need to change the capacity in all nodes after one node
finishs reshape. And as we did before, we can't change the
capacity directly in md_do_sync, instead, the capacity should
be only changed in update_size or received CHANGE_CAPACITY
msg.

So master node calls update_size after completes reshape in
md_reap_sync_thread, but we need to skip ops->update_size if
MD_CLOSING is set since reshaping could not be finish.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>