fss_copy_on_write
fscow_disestablish(sc->sc_mount, fss_copy_on_write, sc);
static int fss_copy_on_write(void *, struct buf *, bool);
error = fscow_establish(sc->sc_mount, fss_copy_on_write, sc);
fscow_disestablish(sc->sc_mount, fss_copy_on_write, sc);