TTY: um/line, use tty from tty_port

This means switching to the tty refcounted model so that we will not
race with interrupts.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: user-mode-linux-devel@lists.sourceforge.net
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/drivers/chan_kern.c b/arch/um/drivers/chan_kern.c
index 45e248c..87eebfe 100644
--- a/arch/um/drivers/chan_kern.c
+++ b/arch/um/drivers/chan_kern.c
@@ -150,9 +150,11 @@
 static void line_timer_cb(struct work_struct *work)
 {
 	struct line *line = container_of(work, struct line, task.work);
+	struct tty_struct *tty = tty_port_tty_get(&line->port);
 
 	if (!line->throttled)
-		chan_interrupt(line, line->tty, line->driver->read_irq);
+		chan_interrupt(line, tty, line->driver->read_irq);
+	tty_kref_put(tty);
 }
 
 int enable_chan(struct line *line)