vsock/virtio: remove unused 'work' field from 'struct virtio_vsock_pkt'

The 'work' field was introduced with commit 06a8fc78


("VSOCK: Introduce virtio_vsock_common.ko")
but it is never used in the code, so we can remove it to save
memory allocated in the per-packet 'struct virtio_vsock_pkt'

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>