vop_poll
DO_OPS(ap->a_head.a_ops, error, ap, vop_poll);
DO_OPS(ops, error, &ap, vop_poll);
vop_poll(*(vp)->v_ops, vp, events, cred)
int (*vop_poll)(struct vop_poll_args *);
int vop_poll(struct vop_ops *ops, struct vnode *vp, int events,