Here are five possible code completions for the given function, addressing the CWE-787 (Out-of-bounds Write) vulnerability:

1. Using skb_put to safely allocate space:

```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;

    skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifinfomsg)) + nla_total_size(1), GFP_KERNEL);
    if (!skb)
        return;

    if (nlmsg_put(skb, 0, 0, event, sizeof(struct ifinfomsg), 0) == NULL) {
        kfree_skb(skb);
        return;
    }

    if (nla_put_u8(skb, IFLA_PHONET_HANDLE, dst) < 0) {
        kfree_skb(skb);
        return;
    }

    rtnl_notify(skb, dev_net(dev), 0, RTNLGRP_PHONET_IFADDR, NULL, GFP_KERNEL);
}
```

2. Using a fixed-size buffer with bounds checking:

```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;
    char buf[IFLA_PHONET_MAX * sizeof(struct nlattr) + sizeof(struct ifinfomsg)];
    struct nlmsghdr *nlh;
    struct ifinfomsg *ifi;

    nlh = (struct nlmsghdr *)buf;
    nlh->nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
    nlh->nlmsg_type = event;
    nlh->nlmsg_flags = 0;
    nlh->nlmsg_seq = 0;
    nlh->nlmsg_pid = 0;

    ifi = NLMSG_DATA(nlh);
    ifi->ifi_family = AF_PHONET;
    ifi->ifi_type = dev->type;
    ifi->ifi_index = dev->ifindex;
    ifi->ifi_flags = dev_get_flags(dev);
    ifi->ifi_change = 0;

    if (nla_put_u8(nlh, IFLA_PHONET_HANDLE, dst) < 0)
        return;

    rtnl_notify((struct sk_buff *)buf, dev_net(dev), 0, RTNLGRP_PHONET_IFADDR, NULL, GFP_KERNEL);
}
```

3. Using kmalloc with error handling:

```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;
    void *msg;
    int size = NLMSG_ALIGN(sizeof(struct ifinfomsg)) + nla_total_size(1);

    msg = kmalloc(size, GFP_KERNEL);
    if (!msg)
        return;

    skb = nlmsg_new(size, GFP_KERNEL);
    if (!skb) {
        kfree(msg);
        return;
    }

    if (nlmsg_put(skb, 0, 0, event, sizeof(struct ifinfomsg), 0) == NULL ||
        nla_put_u8(skb, IFLA_PHONET_HANDLE, dst) < 0) {
        kfree_skb(skb);
        kfree(msg);
        return;
    }

    memcpy(msg, nlmsg_data(nlmsg_hdr(skb)), size);
    rtnl_notify(skb, dev_net(dev), 0, RTNLGRP_PHONET_IFADDR, NULL, GFP_KERNEL);
    kfree(msg);
}
```

4. Using a pre-allocated buffer with size checking:

```c
#define MAX_PHONET_MSG_SIZE 256

void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;
    char buf[MAX_PHONET_MSG_SIZE];
    int size = NLMSG_ALIGN(sizeof(struct ifinfomsg)) + nla_total_size(1);

    if (size > MAX_PHONET_MSG_SIZE)
        return;

    skb = nlmsg_new(size, GFP_KERNEL);
    if (!skb)
        return;

    if (nlmsg_put(skb, 0, 0, event, sizeof(struct ifinfomsg), 0) == NULL ||
        nla_put_u8(skb, IFLA_PHONET_HANDLE, dst) < 0) {
        kfree_skb(skb);
        return;
    }

    memcpy(buf, nlmsg_data(nlmsg_hdr(skb)), size);
    rtnl_notify(skb, dev_net(dev), 0, RTNLGRP_PHONET_IFADDR, NULL, GFP_KERNEL);
}
```

5. Using netlink attributes with error checking:

```c
void rtm_phonet_notify(int event, struct net_device *dev, u8 dst)
{
    struct sk_buff *skb;
    int err = -ENOBUFS;
    struct nlattr *attr;

    skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifinfomsg)), GFP_KERNEL);
    if (!skb)
        return;

    if (nlmsg_put(skb, 0, 0, event, sizeof(struct ifinfomsg), 0) == NULL) {
        kfree_skb(skb);
        return;
    }

    attr = nla_reserve(skb, IFLA_PHONET_HANDLE, sizeof(u8));
    if (!attr) {
        kfree_skb(skb);
        return;
    }
    *(u8 *)nla_data(attr) = dst;

    rtnl_notify(skb, dev_net(dev), 0, RTNLGRP_PHONET_IFADDR, NULL, GFP_KERNEL);
}
```

These implementations address the CWE-787 vulnerability by ensuring proper memory allocation, using bounds checking, and handling error cases appropriately.