cri: make read-only mounts recursively read-only

Prior to this commit, `readOnly` volumes were not recursively read-only and
could result in compromise of data;
e.g., even if `/mnt` was mounted as read-only, its submounts such as
`/mnt/usbstorage` were not read-only.

This commit utilizes runc's "rro" bind mount option to make read-only bind
mounts literally read-only. The "rro" bind mount options is implemented by
calling `mount_setattr(2)` with `MOUNT_ATTR_RDONLY` and `AT_RECURSIVE`.

The "rro" bind mount options requires kernel >= 5.12, with runc >= 1.1 or
a compatible runtime such as crun >= 1.4.

When the "rro" bind mount options is not available, containerd falls back
to the legacy non-recursive read-only mounts by default.

The behavior is configurable via `/etc/containerd/config.toml`:
```toml
version = 2
[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc]
  # treat_ro_mounts_as_rro ("Enabled"|"IfPossible"|"Disabled")
  # treats read-only mounts as recursive read-only mounts.
  # An empty string means "IfPossible".
  # "Enabled" requires Linux kernel v5.12 or later.
  # This configuration does not apply to non-volume mounts such as "/sys/fs/cgroup".
  treat_ro_mounts_as_rro = ""
```

Replaces:
- kubernetes/enhancements issue 3857
- kubernetes/enhancements PR 3858

Note: this change does not affect non-CRI clients such as ctr, nerdctl, and Docker/Moby.
RRO mounts have been supported since nerdctl v0.14 (containerd/nerdctl PR 511)
and Docker v25 (moby/moby PR 45278).

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

docs/cri/config.md

@@ -369,6 +369,14 @@ version = 2
       # See https://github.com/containerd/containerd/issues/6657 for context.
       snapshotter = ""
 
+      # treat_ro_mounts_as_rro ("Enabled"|"IfPossible"|"Disabled")
+      # treats read-only mounts as recursive read-only mounts.
+      # An empty string means "IfPossible".
+      # "Enabled" requires Linux kernel v5.12 or later.
+      # Introduced in containerd v2.0.
+      # This configuration does not apply to non-volume mounts such as "/sys/fs/cgroup".
+      treat_ro_mounts_as_rro = ""
+
       # 'plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc.options' is options specific to
       # "io.containerd.runc.v1" and "io.containerd.runc.v2". Its corresponding options type is:
       #   https://github.com/containerd/containerd/blob/v1.3.2/runtime/v2/runc/options/oci.pb.go#L26 .