xfrm: check for xdo_dev_state_free

The current XFRM code assumes that we've implemented the
xdo_dev_state_free() callback, even if it is meaningless to the driver.
This patch adds a check for it before calling, as done in other APIs,
to prevent a NULL function pointer kernel crash.
Signed-off-by: NShannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1933,7 +1933,8 @@ static inline void xfrm_dev_state_free(struct xfrm_state *x)
 	 struct net_device *dev = xso->dev;
 
 	if (dev && dev->xfrmdev_ops) {
-		dev->xfrmdev_ops->xdo_dev_state_free(x);
+		if (dev->xfrmdev_ops->xdo_dev_state_free)
+			dev->xfrmdev_ops->xdo_dev_state_free(x);
 		xso->dev = NULL;
 		dev_put(dev);
 	}