Storage dm_crypt + preserve Always Fails

Running via Subiquity

In this case, I'm trying to install Ubuntu Server onto a luks btrfs root.

Given the autoinstall storage config:

```
    config:

      - id: sda-disk
        type: disk
        ptable: gpt
        preserve: true
        path: /dev/sda

      - id: efi-partition
        type: partition
        size: 1072693248
        device: sda-disk
        flag: boot
        partition_type: EF00
        preserve: true
        grub_device: true
      - id: boot-partition
        type: partition
        size: 1G
        device: sda-disk
        preserve: true
      - id: root-partition
        type: partition
        size: 134216679424
        device: sda-disk
        preserve: true

      - id: root-crypt
        type: dm_crypt
        dm_name: crypto_root
        volume: root-partition
        key: password
        preserve: true

      - id: root-format
        type: format
        fstype: btrfs
        preserve: true
        volume: root-crypt
      - id: boot-format
        type: format
        fstype: btrfs
        preserve: true
        volume: boot-partition
      - id: efi-format
        type: format
        fstype: fat32
        preserve: true
        volume: efi-partition

      - id: boot-mount
        type: mount
        path: /boot
        device: boot-format
      - id: efi-mount
        type: mount
        path: /boot/efi
        device: efi-format
      - id: root-mount
        type: mount
        path: /
        device: root-format
```

Curtin fails with the following error:

```
Verifying /dev/mapper/crypto_root exists
Running command ['dmsetup', 'info', '/dev/mapper/crypto_root', '-C', '-o', 'name,uuid,blkdevname,blkdevs_used,subsystem', '--noheading', '--separator', '='] with allowed return codes [0] (capture=True)
Verifying /dev/mapper/crypto_root volume, expecting /dev/sda3 , found sda3
An error occured handling 'root-crypt': RuntimeError - Verifying /dev/mapper/crypto_root volume, expecting /dev/sda3 , found sda3
finish: cmd-install/stage-partitioning/builtin/cmd-block-meta: FAIL: configuring dm_crypt: root-crypt
TIMED BLOCK_META: 1.230
finish: cmd-install/stage-partitioning/builtin/cmd-block-meta: FAIL: curtin command block-meta
Traceback (most recent call last):
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/main.py", line 202, in main
    ret = args.func(args)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/log.py", line 97, in wrapper
    return log_time("TIMED %s: " % msg, func, *args, **kwargs)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/log.py", line 79, in log_time
    return func(*args, **kwargs)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/block_meta.py", line 118, in block_meta
    return meta_custom(args)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/block_meta.py", line 2097, in meta_custom
    handler(command, storage_config_dict, context)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/block_meta.py", line 1514, in dm_crypt_handler
    dm_crypt_verify(dmcrypt_dev, volume_path)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/block_meta.py", line 1476, in dm_crypt_verify
    verify_blkdev_used(dmcrypt_dev, volume_path)
  File "/snap/subiquity/4229/lib/python3.8/site-packages/curtin/commands/block_meta.py", line 1471, in verify_blkdev_used
    raise RuntimeError(msg)
RuntimeError: Verifying /dev/mapper/crypto_root volume, expecting /dev/sda3 , found sda3
Verifying /dev/mapper/crypto_root volume, expecting /dev/sda3 , found sda3
```

Note that the command in the logs returns:

```
# dmsetup info /dev/mapper/crypto_root -C -o name,uuid,blkdevname,blkdevs_used,subsystem --noheading --separator =
crypto_root=CRYPT-LUKS2-b2f76b46a21e4cb9b35676a4849c47c0-crypto_root=dm-0=sda3=CRYPT
```

I'm not sure when this code would succeed given the code: https://github.com/canonical/curtin/blob/8c5f87ed567b2a4cdddeb4a49ad607011b34aa49/curtin/commands/block_meta.py#L565-L575

Which makes me think this is likely a bug.