Assemble: provide protection when clustered raid do assemble

Assemble: provide protection when clustered raid do assemble

The previous patch provides protection for other modes
such as CREATE, MANAGE, GROW and INCREMENTAL. And for
ASSEMBLE mode, we also need to protect during the process
of assemble clustered raid.

However, we can only know the array is clustered or not
when the metadata is ready, so the lock_cluster is called
after select_devices(). And we could re-read the metadata
when doing auto-assembly, so refresh the locking.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>