ipv6: fix icmp6_dst_alloc()

commit 87a11578 ( ipv6: Move xfrm_lookup() call down into
icmp6_dst_alloc().) forgot to convert one error path, leading
to crashes in mld_sendpack()

Many thanks to Dave Jones for providing a very complete bug report.
Reported-by: NDave Jones <davej@redhat.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1077,7 +1077,7 @@ struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
 	struct net *net = dev_net(dev);
 
 	if (unlikely(!idev))
-		return NULL;
+		return ERR_PTR(-ENODEV);
 
 	rt = ip6_dst_alloc(&net->ipv6.ip6_dst_ops, dev, 0);
 	if (unlikely(!rt)) {