netrom: fix info leak via msg_name in nr_recvmsg()

In case msg_name is set the sockaddr info gets filled out, as
requested, but the code fails to initialize the padding bytes of
struct sockaddr_ax25 inserted by the compiler for alignment. Also
the sax25_ndigis member does not get assigned, leaking four more
bytes.

Both issues lead to the fact that the code will leak uninitialized
kernel stack bytes in net/socket.c.

Fix both issues by initializing the memory with memset(0).

Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int nr_listen(struct socket *sock, int backlog)
{
	struct sock *sk = sock->sk;

	lock_sock(sk);
	if (sk->sk_state != TCP_LISTEN) {
		memset(&nr_sk(sk)->user_addr, 0, AX25_ADDR_LEN);
		sk->sk_max_ack_backlog = backlog;
		sk->sk_state           = TCP_LISTEN;
		release_sock(sk);
		return 0;
	}
	release_sock(sk);

	return -EOPNOTSUPP;
}

static int nr_listen(struct socket *sock, int backlog)
{
	struct sock *sk = sock->sk;

	lock_sock(sk);
	if (sk->sk_state != TCP_LISTEN) {
		memset(&nr_sk(sk)->user_addr, 0, AX25_ADDR_LEN);
		sk->sk_max_ack_backlog = backlog;
		sk->sk_state           = TCP_LISTEN;
		release_sock(sk);
		return 0;
	}
	release_sock(sk);

	return -EOPNOTSUPP;
}
