mdadm: improve the safeguard for change cluster raid's sb

mdadm: improve the safeguard for change cluster raid's sb

This commit does the following jobs:

1. rename is_clustered to dlm_funs_ready since it match the
   function better.
2. st->cluster_name can't be use to identify the raid is a
   clustered or not, we should check the bitmap's version to
   perform the identification.
3. for cluster_get_dlmlock/cluster_release_dlmlock funcs, both
   of them just need the lockid as parameter since the cluster
   name can get by get_cluster_name().

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