usb: gadget: u_serial: add missing port entry locking

gserial_alloc_line() misses locking (for a release barrier) while
resetting port entry on TTY allocation failure. Fix this.

Cc: stable@vger.kernel.org
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_serial.c

@@ -1239,8 +1239,10 @@ int gserial_alloc_line(unsigned char *line_num)
 				__func__, port_num, PTR_ERR(tty_dev));
 
 		ret = PTR_ERR(tty_dev);
+		mutex_lock(&ports[port_num].lock);
 		port = ports[port_num].port;
 		ports[port_num].port = NULL;
+		mutex_unlock(&ports[port_num].lock);
 		gserial_free_port(port);
 		goto err;
 	}