md-cluster: fix safemode_delay value when converting to clustered bitmap

When array convert to clustered bitmap, the safe_mode_delay doesn't
clean and vice versa. the /sys/block/mdX/md/safe_mode_delay keep original
value after changing bitmap type. In safe_delay_store(), the code forbids
setting mddev->safemode_delay when array is clustered. So in cluster-md
env, the expected safemode_delay value should be 0.

Reproducible steps:
```
node1 # mdadm --zero-superblock /dev/sd{b,c,d}
node1 # mdadm -C /dev/md0 -b internal -e 1.2 -n 2 -l mirror /dev/sdb /dev/sdc
node1 # cat /sys/block/md0/md/safe_mode_delay
0.204
node1 # mdadm -G /dev/md0 -b none
node1 # mdadm --grow /dev/md0 --bitmap=clustered
node1 # cat /sys/block/md0/md/safe_mode_delay
0.204 <== doesn't change, should ZERO for cluster-md

node1 # mdadm --zero-superblock /dev/sd{b,c,d}
node1 # mdadm -C /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sdb /dev/sdc
node1 # cat /sys/block/md0/md/safe_mode_delay
0.000
node1 # mdadm -G /dev/md0 -b none
node1 # cat /sys/block/md0/md/safe_mode_delay
0.000 <== doesn't change, should default value
```

Reviewed-by: NeilBrown <[e-mail]>
Signed-off-by: Zhao Heming <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>

drivers/md/md.c

···
  * count by 2 for every hour elapsed between read errors.
  */
 #define MD_DEFAULT_MAX_CORRECTED_READ_ERRORS 20
+/* Default safemode delay: 200 msec */
+#define DEFAULT_SAFEMODE_DELAY ((200 * HZ)/1000 +1)
 /*
  * Current RAID-1,4,5 parallel reconstruction 'guaranteed speed limit'
  * is 1000 KB/sec, so the extra system load does not show up that much.
···
 	if (mddev_is_clustered(mddev))
 		mddev->safemode_delay = 0;
 	else
-		mddev->safemode_delay = (200 * HZ)/1000 +1; /* 200 msec delay */
+		mddev->safemode_delay = DEFAULT_SAFEMODE_DELAY;
 	mddev->in_sync = 1;
 	smp_wmb();
 	spin_lock(&mddev->lock);
···
 
 				mddev->bitmap_info.nodes = 0;
 				md_cluster_ops->leave(mddev);
+				mddev->safemode_delay = DEFAULT_SAFEMODE_DELAY;
 			}
 			mddev_suspend(mddev);
 			md_bitmap_destroy(mddev);
···
 
 int md_setup_cluster(struct mddev *mddev, int nodes)
 {
+	int ret;
 	if (!md_cluster_ops)
 		request_module("md-cluster");
 	spin_lock(&pers_lock);
···
 	}
 	spin_unlock(&pers_lock);
 
-	return md_cluster_ops->join(mddev, nodes);
+	ret = md_cluster_ops->join(mddev, nodes);
+	if (!ret)
+		mddev->safemode_delay = 0;
+	return ret;
 }
 
 void md_cluster_stop(struct mddev *mddev)