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>
int inet_rcv_saddr_equal(const struct sock *sk, const struct sock *sk2,
			 bool match_wildcard)
{
#if IS_ENABLED(CONFIG_IPV6)
	if (sk->sk_family == AF_INET6)
		return ipv6_rcv_saddr_equal(&sk->sk_v6_rcv_saddr,
					    inet6_rcv_saddr(sk2),
					    sk->sk_rcv_saddr,
					    sk2->sk_rcv_saddr,
					    ipv6_only_sock(sk),
					    ipv6_only_sock(sk2),
					    match_wildcard);
#endif
	return ipv4_rcv_saddr_equal(sk->sk_rcv_saddr, sk2->sk_rcv_saddr,
				    ipv6_only_sock(sk2), match_wildcard);
}

int inet_rcv_saddr_equal(const struct sock *sk, const struct sock *sk2,
			 bool match_wildcard)
{
#if IS_ENABLED(CONFIG_IPV6)
	if (sk->sk_family == AF_INET6)
		return ipv6_rcv_saddr_equal(&sk->sk_v6_rcv_saddr,
					    inet6_rcv_saddr(sk2),
					    sk->sk_rcv_saddr,
					    sk2->sk_rcv_saddr,
					    ipv6_only_sock(sk),
					    ipv6_only_sock(sk2),
					    match_wildcard);
#endif
	return ipv4_rcv_saddr_equal(sk->sk_rcv_saddr, sk2->sk_rcv_saddr,
				    ipv6_only_sock(sk2), match_wildcard);
}
