ipv6: fix dereference of rt6_ex before null check error

Currently rt6_ex is being dereferenced before it is null checked
hence there is a possible null dereference bug. Fix this by only
dereferencing rt6_ex after it has been null checked.

Detected by CoverityScan, CID#1457749 ("Dereference before null check")

Fixes: 81eb8447 ("ipv6: take care of rt6_stats")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1142,10 +1142,12 @@ static DEFINE_SPINLOCK(rt6_exception_lock);
 static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
 				 struct rt6_exception *rt6_ex)
 {
-	struct net *net = dev_net(rt6_ex->rt6i->dst.dev);
+	struct net *net;
 
 	if (!bucket || !rt6_ex)
 		return;
+
+	net = dev_net(rt6_ex->rt6i->dst.dev);
 	rt6_ex->rt6i->rt6i_node = NULL;
 	hlist_del_rcu(&rt6_ex->hlist);
 	rt6_release(rt6_ex->rt6i);