LWB_STATE_CLOSED
EQUIV(lwb->lwb_root_zio == NULL, lwb->lwb_state == LWB_STATE_CLOSED);
ASSERT3S(lwb->lwb_state, !=, LWB_STATE_CLOSED);
IMPLY(lwb != NULL, lwb->lwb_state != LWB_STATE_CLOSED);
lwb->lwb_state = LWB_STATE_CLOSED;
ASSERT(lwb->lwb_state == LWB_STATE_CLOSED ||