vhost: Reintroduce kthread API and add mode selection

Since commit 6e890c5d ("vhost: use vhost_tasks for worker threads"),
the vhost uses vhost_task and operates as a child of the
owner thread. This is required for correct CPU usage accounting,
especially when using containers.

However, this change has caused confusion for some legacy
userspace applications, and we didn't notice until it's too late.

Unfortunately, it's too late to revert - we now have userspace
depending both on old and new behaviour :(

To address the issue, reintroduce kthread mode for vhost workers and
provide a configuration to select between kthread and task worker.

- Add 'fork_owner' parameter to vhost_dev to let users select kthread
  or task mode. Default mode is task mode(VHOST_FORK_OWNER_TASK).

- Reintroduce kthread mode support:
  * Bring back the original vhost_worker() implementation,
    and renamed to vhost_run_work_kthread_list().
  * Add cgroup support for the kthread
  * Introduce struct vhost_worker_ops:
    - Encapsulates create / stop / wake‑up callbacks.
    - vhost_worker_create() selects the proper ops according to
      inherit_owner.

- Userspace configuration interface:
  * New IOCTLs:
      - VHOST_SET_FORK_FROM_OWNER lets userspace select task mode
        (VHOST_FORK_OWNER_TASK) or kthread mode (VHOST_FORK_OWNER_KTHREAD)
      - VHOST_GET_FORK_FROM_OWNER reads the current worker mode
  * Expose module parameter 'fork_from_owner_default' to allow system
    administrators to configure the default mode for vhost workers
  * Kconfig option CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL controls whether
    these IOCTLs and the parameter are available

- The VHOST_NEW_WORKER functionality requires fork_owner to be set
  to true, with validation added to ensure proper configuration

This partially reverts or improves upon:
  commit 6e890c5d ("vhost: use vhost_tasks for worker threads")
  commit 1cdaafa1 ("vhost: replace single worker pointer with xarray")

Fixes: 6e890c5d ("vhost: use vhost_tasks for worker threads"),
Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20250714071333.59794-2-lulu@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>