ipv6: fix a dst leak when removing its exception

These is no need to hold dst before calling rt6_remove_exception_rt().
The call to dst_hold_safe() in ip6_link_failure() was for ip6_del_rt(),
which has been removed in Commit 93531c67 ("net/ipv6: separate
handling of FIB entries from dst based routes"). Otherwise, it will
cause a dst leak.

This patch is to simply remove the dst_hold_safe() call before calling
rt6_remove_exception_rt() and also do the same in ip6_del_cached_rt().
It's safe, because the removal of the exception that holds its dst's
refcnt is protected by rt6_exception_lock.

Fixes: 93531c67 ("net/ipv6: separate handling of FIB entries from dst based routes")
Fixes: 23fb93a4 ("net/ipv6: Cleanup exception and cache route handling")
Reported-by: NLi Shuang <shuali@redhat.com>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2232,8 +2232,7 @@ static void ip6_link_failure(struct sk_buff *skb)
 	if (rt) {
 		rcu_read_lock();
 		if (rt->rt6i_flags & RTF_CACHE) {
-			if (dst_hold_safe(&rt->dst))
-				rt6_remove_exception_rt(rt);
+			rt6_remove_exception_rt(rt);
 		} else {
 			struct fib6_info *from;
 			struct fib6_node *fn;
@@ -3214,8 +3213,8 @@ static int ip6_del_cached_rt(struct rt6_info *rt, struct fib6_config *cfg)
 	if (cfg->fc_flags & RTF_GATEWAY &&
 	    !ipv6_addr_equal(&cfg->fc_gateway, &rt->rt6i_gateway))
 		goto out;
-	if (dst_hold_safe(&rt->dst))
-		rc = rt6_remove_exception_rt(rt);
+
+	rc = rt6_remove_exception_rt(rt);
 out:
 	return rc;
 }