net: use rcu_dereference_protected to fetch sk_dst_cache in sk_destruct

As Eric noticed, in .sk_destruct, sk->sk_dst_cache update is prevented, and
no barrier is needed for this. So change to use rcu_dereference_protected()
instead of rcu_dereference_check() to fetch sk_dst_cache in there.

v1->v2:
  - no change, repost after net-next is open.
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/af_decnet.c

@@ -444,7 +444,7 @@ static void dn_destruct(struct sock *sk)
 	skb_queue_purge(&scp->other_xmit_queue);
 	skb_queue_purge(&scp->other_receive_queue);
 
-	dst_release(rcu_dereference_check(sk->sk_dst_cache, 1));
+	dst_release(rcu_dereference_protected(sk->sk_dst_cache, 1));
 }
 
 static unsigned long dn_memory_pressure;

net/ipv4/af_inet.c

@@ -160,7 +160,7 @@ void inet_sock_destruct(struct sock *sk)
 	WARN_ON(sk->sk_forward_alloc);
 
 	kfree(rcu_dereference_protected(inet->inet_opt, 1));
-	dst_release(rcu_dereference_check(sk->sk_dst_cache, 1));
+	dst_release(rcu_dereference_protected(sk->sk_dst_cache, 1));
 	dst_release(sk->sk_rx_dst);
 	sk_refcnt_debug_dec(sk);
 }