USB: cdc-acm: more sanity checking

An attack has become available which pretends to be a quirky
device circumventing normal sanity checks and crashes the kernel
by an insufficient number of interfaces. This patch adds a check
to the code path for quirky devices.

Signed-off-by: Oliver Neukum <ONeukum@suse.com>
CC: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static int acm_tty_write_room(struct tty_struct *tty)
{
	struct acm *acm = tty->driver_data;
	/*
	 * Do not let the line discipline to know that we have a reserve,
	 * or it might get too enthusiastic.
	 */
	return acm_wb_is_avail(acm) ? acm->writesize : 0;
}

static int acm_tty_write_room(struct tty_struct *tty)
{
	struct acm *acm = tty->driver_data;
	/*
	 * Do not let the line discipline to know that we have a reserve,
	 * or it might get too enthusiastic.
	 */
	return acm_wb_is_avail(acm) ? acm->writesize : 0;
}
