tcp: fix possible socket refcount problem for ipv6

commit 144d56e91044181ec0ef67aeca91e9a8b5718348
("tcp: fix possible socket refcount problem") is missing
the IPv6 part. As tcp_release_cb is shared by both protocols
we should hold sock reference for the TCP_MTU_REDUCED_DEFERRED
bit.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -403,8 +403,9 @@ static void tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		tp->mtu_info = ntohl(info);
 		if (!sock_owned_by_user(sk))
 			tcp_v6_mtu_reduced(sk);
-		else
-			set_bit(TCP_MTU_REDUCED_DEFERRED, &tp->tsq_flags);
+		else if (!test_and_set_bit(TCP_MTU_REDUCED_DEFERRED,
+					   &tp->tsq_flags))
+			sock_hold(sk);
 		goto out;
 	}