Here are five possible code completions for the given function:

**Option 1: Allocate a new skb and handle errors**
```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = alloc_skb(MAX_SKB_SIZE, GFP_KERNEL);
    if (!skb) {
        printk(KERN_ERR "Failed to allocate skb\n");
        return;
    }

    // ... rest of the function ...
}
```

**Option 2: Allocate a new skb with a specific size and handle errors**
```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = alloc_skb(sizeof(struct phonet_msg) + dev->mtu, GFP_KERNEL);
    if (!skb) {
        printk(KERN_ERR "Failed to allocate skb\n");
        return;
    }

    // ... rest of the function ...
}
```

**Option 3: Allocate a new skb and handle errors with a specific error code**
```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = alloc_skb(MAX_SKB_SIZE, GFP_KERNEL);
    if (!skb) {
        err = -ENOMEM;
        printk(KERN_ERR "Failed to allocate skb: %d\n", err);
        return;
    }

    // ... rest of the function ...
}
```

**Option 4: Allocate a new skb and handle errors with a specific error code and logging**
```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = alloc_skb(MAX_SKB_SIZE, GFP_KERNEL);
    if (!skb) {
        err = -ENOMEM;
        netdev_err(dev, "Failed to allocate skb: %d\n", err);
        return;
    }

    // ... rest of the function ...
}
```

**Option 5: Allocate a new skb and handle errors with a specific error code, logging, and a fallback**
```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = alloc_skb(MAX_SKB_SIZE, GFP_KERNEL);
    if (!skb) {
        err = -ENOMEM;
        netdev_err(dev, "Failed to allocate skb: %d\n", err);
        // Fallback to a smaller skb size
        skb = alloc_skb(MAX_SKB_SIZE / 2, GFP_KERNEL);
        if (!skb) {
            printk(KERN_ERR "Failed to allocate fallback skb\n");
            return;
        }
    }

    // ... rest of the function ...
}
```