[devserver][clean_disk] make antlir2-out cleaner more resilient

Summary:
The top level `antlir2-out` dir has not held subvolumes since D76380320.
But there might still be devservers that have subvols in the old location. This
just cleans up this script to handle

Test Plan:
Created a convoluted setup with subvols in both `antlir2-out/` and
`antlir2-out/subvols`, including nested subvolumes in read-only parents. This
new script still cleaned it up.
```
[email protected] in fbsource/antlir2-out
❯ btrfs subvolume create foo
Create subvolume './foo'

[email protected] in fbsource/antlir2-out
❯ btrfs subvolume create subvols/bar
Create subvolume 'subvols/bar'

[email protected] in fbsource/antlir2-out
❯ btrfs subvolume create subvols/bar/baz
Create subvolume 'subvols/bar/baz'

[email protected] in fbsource/antlir2-out
❯ sudo btrfs property set subvols/bar ro true

[email protected] in fbsource/antlir2-out
❯ tree
.
├── foo
└── subvols
    └── bar
        └── baz

4 directories, 0 files

[email protected] in fbsource/antlir2-out
❯ ~fbcode/antlir/clean-antlir2-out.sh
+ '[' '!' -d antlir2-out ']'
+ '[' '!' -d antlir2-out ']'
+ pushd antlir2-out
/data/users/vmagro/fbsource/antlir2-out /data/users/vmagro/fbsource
+ find . -maxdepth 2 -depth -type d -inum 256 -exec sudo btrfs property set '{}' ro false ';'
+ sudo find . -depth -type d -inum 256 -exec btrfs subvolume delete '{}' ';'
Delete subvolume 3169 (no-commit): '/data/users/vmagro/fbsource/antlir2-out/subvols/bar/baz'
Delete subvolume 3168 (no-commit): '/data/users/vmagro/fbsource/antlir2-out/subvols/bar'
Delete subvolume 3167 (no-commit): '/data/users/vmagro/fbsource/antlir2-out/foo'

[email protected] in fbsource/antlir2-out
❯ tree
.
└── subvols

1 directory, 0 files
```

Rollback Plan:

Reviewed By: sergeyfd

Differential Revision: D81242825

fbshipit-source-id: abf6737aa74418c8339a08f526941ad6d4b0c536

Author: vmagro

antlir/clean-antlir2-out.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/bash
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+set -ex
+
+if [ ! -d "antlir2-out" ]; then
+    cd "$(hg root)"
+fi
+
+if [ ! -d "antlir2-out" ]; then
+    echo "no antlir2-out found in repo root, exiting..."
+    exit
+fi
+
+pushd "antlir2-out"
+
+# all top-level subvolumes must be made read-write in case any nested
+# subvolume needs to be deleted (the compiler will now loudly fail if these are
+# found, but we still want to clean them up)
+# subvolumes may be found in either antlir2-out (old style) or
+# antlir2-out/subvols (new style) hence the -maxdepth 2
+find . -maxdepth 2 -depth -type d -inum 256 -exec sudo btrfs property set {} ro false \;
+
+# delete any nested subvolumes (sudo because we might not have permissions to
+# read the directories that they're found in)
+sudo find . -depth -type d -inum 256 -exec btrfs subvolume delete {} \;