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

1. Option 1:
```c
static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
{
    if (skb && len <= skb->head - skb->data) {
        skb->data -= len;
        skb->len  += len;
        return skb->data;
    }
    return NULL;
}
```

2. Option 2:
```c
static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
{
    if (skb && skb->data - len >= skb->head) {
        skb->data -= len;
        skb->len  += len;
        return skb->data;
    }
    return NULL;
}
```

3. Option 3:
```c
static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
{
    if (skb && len > 0 && skb->data - len >= skb->head) {
        skb->data -= len;
        skb->len  += len;
        return skb->data;
    }
    return NULL;
}
```

4. Option 4:
```c
static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
{
    if (!skb || len > skb_headroom(skb))
        return NULL;
    
    skb->data -= len;
    skb->len  += len;
    return skb->data;
}
```

5. Option 5:
```c
static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
{
    if (skb && len <= skb_headroom(skb)) {
        skb->data -= len;
        skb->len  += len;
        skb->mac_header = (skb->mac_header == ~0U) ? ~0U : skb->mac_header - len;
        return skb->data;
    }
    return NULL;
}
```