USB: cdc-acm: fix potential urb leak and PM imbalance in write

commit 183a4508 upstream.

Make sure to check return value of autopm get in write() in order to
avoid urb leak and PM counter imbalance on errors.

Fixes: 11ea859d ("USB: additional power savings for cdc-acm devices
that support remote wakeup")
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2:
 - Adjust context
 - Error/status variable is called rc, not stat]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/class/cdc-acm.c

@@ -191,7 +191,13 @@ static int acm_write_start(struct acm *acm, int wbn)
 
 	dev_vdbg(&acm->data->dev, "%s - susp_count %d\n", __func__,
 							acm->susp_count);
-	usb_autopm_get_interface_async(acm->control);
+	rc = usb_autopm_get_interface_async(acm->control);
+	if (rc) {
+		wb->use = 0;
+		spin_unlock_irqrestore(&acm->write_lock, flags);
+		return rc;
+	}
+
 	if (acm->susp_count) {
 		usb_anchor_urb(wb->urb, &acm->delayed);
 		spin_unlock_irqrestore(&acm->write_lock, flags);