tty: serial: lpuart: avoid leaking struct tty_struct

The function tty_port_tty_get() gets a reference to the tty. Since
the code is not using tty_port_tty_set(), the reference is kept
even after closing the tty.

Avoid using tty_port_tty_get() by directly access the tty instance.
Since lpuart_start_rx_dma() is called from the .startup() and
.set_termios() callback, it is safe to assume the tty instance is
valid.

Cc: stable@vger.kernel.org # v4.9+
Fixes: 5887ad43 ("tty: serial: fsl_lpuart: Use cyclic DMA for Rx")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -979,7 +979,8 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
 	struct circ_buf *ring = &sport->rx_ring;
 	int ret, nent;
 	int bits, baud;
-	struct tty_struct *tty = tty_port_tty_get(&sport->port.state->port);
+	struct tty_port *port = &sport->port.state->port;
+	struct tty_struct *tty = port->tty;
 	struct ktermios *termios = &tty->termios;
 
 	baud = tty_get_baud_rate(tty);