Due to a bug in commit 9c44407, when the user and mount namespaces
are used, and the bind mount is followed by the cgroup mount in the
spec, the cgroup is mounted using the bind mount's mount fd.

This can be reproduced with podman 4.1 (when configured to use runc):

$ podman run --uidmap 0:100:10000 quay.io/libpod/testimage:20210610 mount
Error: /home/kir/git/runc/runc: runc create failed: unable to start container process: error during container init: error mounting "cgroup" to rootfs at "/sys/fs/cgroup": mount /proc/self/fd/11:/sys/fs/cgroup/systemd (via /proc/self/fd/12), flags: 0x20502f: operation not permitted: OCI permission denied

or manually with the spec mounts containing something like this:

    {
      "destination": "/etc/resolv.conf",
      "type": "bind",
      "source": "/userdata/resolv.conf",
      "options": [
        "bind"
      ]
    },
    {
      "destination": "/sys/fs/cgroup",
      "type": "cgroup",
      "source": "cgroup",
      "options": [
        "rprivate",
        "nosuid",
        "noexec",
        "nodev",
        "relatime",
        "ro"
      ]
    }

The issue was not found earlier since it requires using userns, and even then
mount fd is ignored by mountToRootfs, except for bind mounts, and all the bind
mounts have mountfd set, except for the case of cgroup v1's /sys/fs/cgroup
which is internally transformed into a bunch of bind mounts.

This is a minimal fix for the issue, suitable for backporting.

A test case is added which reproduces the issue without the fix applied.

Fixes: 9c44407 ("Open bind mount sources from the host userns")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit d370e3c)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>