Here are five possible code completions for the given function, addressing the CWE-190 (Integer Overflow or Wraparound) vulnerability:

1. Using size_t for size calculations:

```c
    size_t payload_size = 0;
    if (prep->datalen > 0) {
        payload_size = prep->datalen;
        if (payload_size > KEY_PAYLOAD_MAX_SIZE) {
            return -EINVAL;
        }
    }
    
    ret = key_payload_reserve(key, payload_size);
    if (ret < 0)
        return ret;
```

2. Implementing bounds checking:

```c
    if (prep->datalen > KEY_PAYLOAD_MAX_SIZE || prep->datalen < 0) {
        return -EINVAL;
    }
    
    ret = key_payload_reserve(key, (size_t)prep->datalen);
    if (ret < 0)
        return ret;
```

3. Using unsigned types and explicit conversion:

```c
    unsigned long payload_size = 0;
    if (prep->datalen > 0) {
        payload_size = (unsigned long)prep->datalen;
        if (payload_size > (unsigned long)KEY_PAYLOAD_MAX_SIZE) {
            return -EINVAL;
        }
    }
    
    ret = key_payload_reserve(key, (size_t)payload_size);
    if (ret < 0)
        return ret;
```

4. Implementing a custom safe addition function:

```c
    int safe_add(int a, int b, int *result) {
        if (a > 0 && b > INT_MAX - a) return -1;
        if (a < 0 && b < INT_MIN - a) return -1;
        *result = a + b;
        return 0;
    }

    int total_size;
    if (safe_add(prep->datalen, sizeof(struct key), &total_size) < 0 || 
        total_size > KEY_PAYLOAD_MAX_SIZE) {
        return -EINVAL;
    }
    
    ret = key_payload_reserve(key, (size_t)prep->datalen);
    if (ret < 0)
        return ret;
```

5. Using GCC's built-in overflow checking functions:

```c
    #include <limits.h>

    int total_size;
    if (__builtin_add_overflow(prep->datalen, sizeof(struct key), &total_size) ||
        total_size > KEY_PAYLOAD_MAX_SIZE) {
        return -EINVAL;
    }
    
    ret = key_payload_reserve(key, (size_t)prep->datalen);
    if (ret < 0)
        return ret;
```

Each of these completions aims to prevent integer overflow by using different techniques such as using appropriate data types, bounds checking, safe arithmetic operations, or compiler-specific functions.