drm/virtio: reclaim pending vbufs before tearing down vqs

virtio_gpu_free_vbufs() destroys the vbufs kmem_cache after the virtqueues
have already been released. Commands that were queued but never completed
by the device leave their vbuffers stranded in the virtqueue, so the cache
still holds live objects when virtio_gpu_deinit() tears everything down.
This triggers a WARNING in virtio_gpu_free_vbufs:

    BUG virtio-gpu-vbufs (Not tainted): Objects remaining in cache
    on __kmem_cache_shutdown()

Drain any buffers still sitting in the control and cursor virtqueues in
virtio_gpu_deinit() after the device has been reset and before the
virtqueues are deleted, following the same pattern used by virtio_console's
remove_vqs(). Each reclaimed buffer is released with free_vbuf(), dropping
the reference on any GEM objects it holds. Pending RESOURCE_UNREF
commands are handled as well: their resp_cb_data still references a GEM
object, so it is cleaned up with virtio_gpu_cleanup_object() to avoid
leaking it on teardown.

Reported-by: syzbot+06f9b2a53ba4a5a47644@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=06f9b2a53ba4a5a47644
Signed-off-by: Anuj Bolewar <bolewara@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20260802-virtio-gpu-reclaim-vbufs-v2-1-5767fb860691@gmail.com

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