mdadm: add the ability to change cluster name

mdadm: add the ability to change cluster name

To support change the cluster name, the commit do the followings:

1. extend original write_bitmap function for new scenario.
2. add the scenarion to handle the modification of cluster's name
   in write_bitmap1.
3. let the cluster name also show in examine_super1 and detail_super1

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.de>