btrfs: disable the device add feature for temp-fsid

The device addition operation will transform the cloned temp-fsid mounted
device into a multi-device filesystem. Therefore, it is marked as
unsupported.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>