Now that there are no more data path users of vhost_vsock_lock, it can
be turned into a mutex.  It's only used by .release() and in the
.ioctl() path.

Depends-on: <20181105103547.22018-1-stefanha@redhat.com>
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>