xfrm6: remove BUG_ON from xfrm6_dst_ifdown

if loopback_idev is NULL pointer, and the following access of
loopback_idev will trigger panic, which is same as BUG_ON
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_policy.c

@@ -262,7 +262,6 @@ static void xfrm6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 	if (xdst->u.rt6.rt6i_idev->dev == dev) {
 		struct inet6_dev *loopback_idev =
 			in6_dev_get(dev_net(dev)->loopback_dev);
-		BUG_ON(!loopback_idev);
 
 		do {
 			in6_dev_put(xdst->u.rt6.rt6i_idev);