btrfs: use num_device to check for the last surviving seed device

For both sprout and seed fsids,
 btrfs_fs_devices::num_devices provides device count including missing
 btrfs_fs_devices::open_devices provides device count excluding missing

We create a dummy struct btrfs_device for the missing device, so
num_devices != open_devices when there is a missing device.

In btrfs_rm_devices() we wrongly check for %cur_devices->open_devices
before freeing the seed fs_devices. Instead we should check for
%cur_devices->num_devices.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2211,7 +2211,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 	synchronize_rcu();
 	btrfs_free_device(device);
 
-	if (cur_devices->open_devices == 0) {
+	if (cur_devices->num_devices == 0) {
 		list_del_init(&cur_devices->seed_list);
 		close_fs_devices(cur_devices);
 		free_fs_devices(cur_devices);