md-cluster: fix rmmod issue when md_cluster convert bitmap to none

update_array_info misses calling module_put when removing cluster bitmap.

steps to reproduce:
```
node1 # mdadm -C /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sda
/dev/sdb
mdadm: array /dev/md0 started.
node1 # lsmod | egrep "dlm|md_|raid1"
md_cluster 28672 1
dlm 212992 14 md_cluster
configfs 57344 2 dlm
raid1 53248 1
md_mod 176128 2 raid1,md_cluster
node1 # mdadm -G /dev/md0 -b none
node1 # lsmod | egrep "dlm|md_|raid1"
md_cluster 28672 1 <== should be zero
dlm 212992 9 md_cluster
configfs 57344 2 dlm
raid1 53248 1
md_mod 176128 2 raid1,md_cluster
node1 # mdadm -G /dev/md0 -b clustered
node1 # lsmod | egrep "dlm|md_|raid1"
md_cluster 28672 2 <== increase
dlm 212992 14 md_cluster
configfs 57344 2 dlm
raid1 53248 1
md_mod 176128 2 raid1,md_cluster
node1 # mdadm -G /dev/md0 -b none
node1 # mdadm -G /dev/md0 -b clustered
node1 # lsmod | egrep "dlm|md_|raid1"
md_cluster 28672 3 <== increase
dlm 212992 14 md_cluster
configfs 57344 2 dlm
raid1 53248 1
md_mod 176128 2 raid1,md_cluster
```

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

drivers/md/md.c

···
 
 				mddev->bitmap_info.nodes = 0;
 				md_cluster_ops->leave(mddev);
+				module_put(md_cluster_mod);
 				mddev->safemode_delay = DEFAULT_SAFEMODE_DELAY;
 			}
 			mddev_suspend(mddev);