tty: n_hdlc not atomic use tty->flags.

N_HDLC can spoil tty->flags because use not atomic operations on tty->flags.
I use n_hdlc line discipline and it happens.

Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/n_hdlc.c

@@ -417,7 +417,7 @@ static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
 				__FILE__,__LINE__,tbuf,tbuf->count);
 
 		/* Send the next block of data to device */
-		tty->flags |= (1 << TTY_DO_WRITE_WAKEUP);
+		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 		actual = tty->ops->write(tty, tbuf->buf, tbuf->count);
 
 		/* rollback was possible and has been done */
@@ -459,7 +459,7 @@ static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
 	}
 
 	if (!tbuf)
-		tty->flags  &= ~(1 << TTY_DO_WRITE_WAKEUP);
+		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 
 	/* Clear the re-entry flag */
 	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
@@ -491,7 +491,7 @@ static void n_hdlc_tty_wakeup(struct tty_struct *tty)
 		return;
 
 	if (tty != n_hdlc->tty) {
-		tty->flags &= ~(1 << TTY_DO_WRITE_WAKEUP);
+		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 		return;
 	}