TTY: serial, use tty_port_close_start helper

After the previous patches, the code is almost identical. There are
few differences in the helper code:
1) flush_buffer when flow_stopped
   * when a user doesn't care about the data, delete it anyways
2) ASYNCB_INITIALIZED test before wait_until_sent_from
   * obviously, there is nothing to wait for if the port is dead
3) drain_delay wait
   * we don't set drain_delay

So we can use the helper now. It indeed removes a bunch of duplicated
code.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/tty/serial/serial_core.c

@@ -1267,46 +1267,8 @@ static void uart_close(struct tty_struct *tty, struct file *filp)
 
 	pr_debug("uart_close(%d) called\n", uport->line);
 
-	spin_lock_irqsave(&port->lock, flags);
-
-	if (tty_hung_up_p(filp)) {
-		spin_unlock_irqrestore(&port->lock, flags);
+	if (tty_port_close_start(port, tty, filp) == 0)
 		return;
-	}
-
-	if ((tty->count == 1) && (port->count != 1)) {
-		/*
-		 * Uh, oh.  tty->count is 1, which means that the tty
-		 * structure will be freed.  port->count should always
-		 * be one in these conditions.  If it's greater than
-		 * one, we've got real problems, since it means the
-		 * serial port won't be shutdown.
-		 */
-		printk(KERN_ERR "uart_close: bad serial port count; tty->count is 1, "
-		       "port->count is %d\n", port->count);
-		port->count = 1;
-	}
-	if (--port->count < 0) {
-		printk(KERN_ERR "uart_close: bad serial port count for %s: %d\n",
-		       tty->name, port->count);
-		port->count = 0;
-	}
-	if (port->count) {
-		spin_unlock_irqrestore(&port->lock, flags);
-		return;
-	}
-
-	/*
-	 * Now we wait for the transmit buffer to clear; and we notify
-	 * the line discipline to only process XON/XOFF characters by
-	 * setting tty->closing.
-	 */
-	set_bit(ASYNCB_CLOSING, &port->flags);
-	tty->closing = 1;
-	spin_unlock_irqrestore(&port->lock, flags);
-
-	if (port->closing_wait != ASYNC_CLOSING_WAIT_NONE)
-		tty_wait_until_sent_from_close(tty, port->closing_wait);
 
 	/*
 	 * At this point, we stop accepting input.  To do this, we