snull fix

I a question about snull (from Doug Winterburn) on this mailing list
back in December. I just figured out how to make it work under 2.2 and
I thought you might want to know. BTW, snull is a sample program in
Rubini's device drivers book.

The key thing is to set skb->pkt_type in snull_rx, before calling
netif_rx:

    /* Write metadata, and then pass to the receive level */
    skb->dev = dev;
    skb->protocol = eth_type_trans(skb, dev);
    skb->ip_summed = CHECKSUM_UNNECESSARY;

    // YOU MUST SET THIS!
    // This is the thing that makes it work under 2.2
    skb->pkt_type = PACKET_HOST;

There's a couple of other tweaks required to get it to compile but I
assume you know about these since you did compile it.