To generate the patch for the given code slice, we need to follow a systematic approach to identify the root cause of the vulnerability and then apply an appropriate fix. Here are the reasoning steps:

### Step 1: Root Cause Analysis

1. **Understanding the Code Context:**
   - The function `do_ipv6_getsockopt` is responsible for handling IPv6 socket options.
   - The variable `len` is used to store the length of the option value.
   - The function interacts with user space through `optval` and `optlen` pointers.

2. **Identifying the Vulnerable Behavior:**
   - The line `return put_user(len, optlen);` is responsible for copying the value of `len` to the user space pointer `optlen`.
   - The function `put_user` is used to safely copy data from kernel space to user space.

3. **Potential Vulnerability:**
   - If `len` is negative, it could lead to an out-of-bounds write when `put_user` attempts to copy the value to user space.
   - Negative values for `len` are not valid for the length of an option value and could indicate an error condition.

4. **Error Handling:**
   - The function should handle cases where `len` is negative by returning an appropriate error code instead of attempting to copy the value to user space.

### Step 2: Fixing Strategy

1. **Adding a Check for Negative `len`:**
   - Before calling `put_user`, we need to check if `len` is negative.
   - If `len` is negative, the function should return the error code directly.

2. **Implementing the Fix:**
   - Add a conditional check to verify if `len` is less than 0.
   - If the condition is true, return `len` as the error code.
   - Otherwise, proceed with the `put_user` call to copy the value to user space.

### Conclusion

The patch ensures that the function does not attempt to copy a negative length value to user space, thereby preventing the out-of-bounds write vulnerability. The patch is as follows:

```diff
<         release_sock(sk);
<         return put_user(len, optlen);
---
>         release_sock(sk);
>         if (len < 0)
>         {
>             return len;
>         }
>         return put_user(len, optlen);
```

### Detailed Reasoning Steps for the Patch:

1. **Release the Socket Lock:**
   - The line `release_sock(sk);` is retained to ensure the socket lock is released before returning from the function.

2. **Check for Negative `len`:**
   - The condition `if (len < 0)` checks if `len` is negative.
   - If `len` is negative, the function returns `len` as the error code, preventing further execution.

3. **Copy Length to User Space:**
   - If `len` is not negative, the function proceeds to call `put_user(len, optlen);` to safely copy the length value to the user space pointer `optlen`.

By following these steps, the patch effectively mitigates the CWE-787 (Out-of-bounds Write) vulnerability in the code.