To generate the patch for the given code slice with a CWE-787 vulnerability, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `len`:
  - `len` is a parameter passed to the function `btrfs_extent_same`.
  - We need to consider what values `len` might take.
  - If `len` is 0, this could lead to an out-of-bounds access or incorrect behavior in the `btrfs_double_lock` function.

- 2: Function Call and Parameters:
  - The `btrfs_double_lock` function is called with `len` as one of its parameters.
  - The function's behavior when `len` is 0 is not explicitly handled in the given code slice.

- 3: Potential Vulnerability:
  - The vulnerability arises if `len` is 0, which could result in incorrect behavior or out-of-bounds access in the `btrfs_double_lock` function.
  - This is a classic example of a potential buffer overflow or incorrect behavior vulnerability. Passing a length of 0 to a function that expects a valid length can lead to unexpected behavior or crashes.

### Conclusion:
The vulnerable behavior in this code is a potential buffer overflow or incorrect behavior. It occurs because the code does not handle the case where `len` is 0 before calling the `btrfs_double_lock` function. If `len` is 0, the function may behave incorrectly or access memory out of bounds, leading to potential crashes or security vulnerabilities. Proper bounds checking should be added to ensure `len` is not 0 before calling the function.

### Step 2: Fixing Strategy

A common fixing strategy is to add a check for the length `len` before calling the `btrfs_double_lock` function. This can be done by adding a conditional statement to return immediately if `len` is 0, avoiding the potential vulnerability. Therefore, the patch should be:
```
<     }
<     btrfs_double_lock(src, loff, dst, dst_loff, len);
---
>     }
>     if (len == 0)
>     {
>         return 0;
>     }
>     btrfs_double_lock(src, loff, dst, dst_loff, len);
```
This patch adds a simple check for `len` being 0 and returns immediately if true, preventing the potential vulnerability.