SS_FIL_DEFER
if (nso->so_state & SS_FIL_DEFER) {
ASSERT(so->so_state & SS_FIL_DEFER);
so->so_state &= ~SS_FIL_DEFER;
ASSERT(so->so_state & SS_FIL_DEFER);
!(so->so_state & SS_FIL_DEFER))
so->so_state |= SS_FIL_DEFER;