tty: Reset itty for other pty

Commit 19ffd68f816878aed456d5e87697f43bd9e3bd2b
('pty: Remove redundant itty reset') introduced a regression
whereby the other pty's linkage is not cleared on teardown.
This triggers a false positive diagnostic in testing.

Properly reset the itty linkage.

Signed-off-by: Peter Hurley <[e-mail]>
Cc: stable <[e-mail]> # 3.10
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_io.c

···
 	tty_free_termios(tty);
 	tty_driver_remove_tty(tty->driver, tty);
 	tty->port->itty = NULL;
+	if (tty->link)
+		tty->link->port->itty = NULL;
 	cancel_work_sync(&tty->port->buf.work);
 
 	if (tty->link)