tty: Add driver unthrottle in ioctl(...,TCFLSH,..).

Regression 'tty: fix "IRQ45: nobody cared"' Regression commit 7b292b4bf9a9d6098440d85616d6ca4c608b8304

Function reset_buffer_flags() also invoked during the ioctl(...,TCFLSH,..).
At the time of request we can have full buffers and throttled driver too. If we don't unthrottle driver, we can get forever throttled driver, because, after request, we will have empty buffers and throttled driver and there is no place to unthrottle driver.
It simple reproduce with "pty" pair then one side sleep on tty->write_wait, and other side do ioctl(...,TCFLSH,..). Then there is no place to do writers wake up.

---

index 12b1fa0..a038be1 100644
@@ -1098,10 +1098,12 @@ int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
        case TCIFLUSH:
                if (ld && ld->ops->flush_buffer)
                        ld->ops->flush_buffer(tty);
+               tty_unthrottle(tty);
                break;
        case TCIOFLUSH:
                if (ld && ld->ops->flush_buffer)
                        ld->ops->flush_buffer(tty);
+                tty_unthrottle(tty);
                /* fall through */
        case TCOFLUSH:
                tty_driver_flush_buffer(tty);