dccp/tcp: do not inherit mc_list from parent

syzkaller found a way to trigger double frees from ip_mc_drop_socket()

It turns out that leave a copy of parent mc_list at accept() time,
which is very bad.

Very similar to commit 8b485ce69876 ("tcp: do not inherit
fastopen_req from parent")

Initial report from Pray3r, completed by Andrey one.
Thanks a lot to them !

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Pray3r <pray3r.z@gmail.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
struct sock *inet_csk_complete_hashdance(struct sock *sk, struct sock *child,
					 struct request_sock *req, bool own_req)
{
	if (own_req) {
		inet_csk_reqsk_queue_drop(sk, req);
		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
		if (inet_csk_reqsk_queue_add(sk, req, child))
			return child;
	}
	/* Too bad, another child took ownership of the request, undo. */
	bh_unlock_sock(child);
	sock_put(child);
	return NULL;
}

struct sock *inet_csk_complete_hashdance(struct sock *sk, struct sock *child,
					 struct request_sock *req, bool own_req)
{
	if (own_req) {
		inet_csk_reqsk_queue_drop(sk, req);
		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
		if (inet_csk_reqsk_queue_add(sk, req, child))
			return child;
	}
	/* Too bad, another child took ownership of the request, undo. */
	bh_unlock_sock(child);
	sock_put(child);
	return NULL;
}
