Step 1: analyze the root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice, let's examine the relevant parts:

Root Cause Analysis:

1. The vulnerability occurs on line 14:
   ```c
   14 	simple_recursive_removal(dir, NULL);
   ```

2. The `dir` variable is used here without proper null checking.

3. The `dir` variable is assigned a value on line 8:
   ```c
   8 	dir = lookup_one_len_unlocked(unit, sb->s_root, strlen(unit));
   ```

4. On line 10, there's a check for an error condition:
   ```c
   10 	if (IS_ERR(dir)) {
   ```

5. However, this check only verifies if `dir` is an error value, not if it's NULL.

6. The code doesn't handle the case where `lookup_one_len_unlocked()` might return NULL.

7. If `lookup_one_len_unlocked()` returns NULL, the code will proceed to line 14 without any safeguards.

8. Calling `simple_recursive_removal()` with a NULL `dir` argument can lead to a null pointer dereference.

The root cause of this vulnerability is the incomplete error checking after the `lookup_one_len_unlocked()` call. While the code checks for error conditions using IS_ERR(), it fails to account for the possibility of a NULL return value. This oversight allows a potential NULL pointer to be passed to `simple_recursive_removal()`, which could lead to a crash or undefined behavior when attempting to use the NULL pointer.

To fix this vulnerability, the code should include an additional check for NULL before using the `dir` variable:

```c
if (IS_ERR(dir) || !dir) {
    // Handle error or return
    return -ENOENT;
}
```

This additional check would prevent the code from proceeding with a NULL `dir` value, thus avoiding the potential null pointer dereference in `simple_recursive_removal()`.
Based on the root cause analysis, here are five possible patches to address the CWE-476 vulnerability:

Patch 1: Add a NULL check before calling simple_recursive_removal
```
<     simple_recursive_removal(dir, NULL);
---
>     if (dir)
>         simple_recursive_removal(dir, NULL);
>     else
>         return -ENOENT;
```

Patch 2: Combine the error and NULL checks
```
< 	if (IS_ERR(dir)) {
< 		return PTR_ERR(dir);
< 	}
< 	simple_recursive_removal(dir, NULL);
---
> 	if (IS_ERR(dir) || !dir) {
> 		return IS_ERR(dir) ? PTR_ERR(dir) : -ENOENT;
> 	}
> 	simple_recursive_removal(dir, NULL);
```

Patch 3: Use a goto statement for error handling
```
< 	if (IS_ERR(dir)) {
< 		return PTR_ERR(dir);
< 	}
< 	simple_recursive_removal(dir, NULL);
---
> 	if (IS_ERR(dir) || !dir) {
> 		ret = IS_ERR(dir) ? PTR_ERR(dir) : -ENOENT;
> 		goto out;
> 	}
> 	simple_recursive_removal(dir, NULL);
> out:
> 	return ret;
```

Patch 4: Use a separate function for the removal operation
```
< 	simple_recursive_removal(dir, NULL);
---
> 	if (dir)
> 		perform_recursive_removal(dir);
> 	else
> 		return -ENOENT;
> 
> 	// Define the new function elsewhere:
> 	// static void perform_recursive_removal(struct dentry *dir)
> 	// {
> 	//     simple_recursive_removal(dir, NULL);
> 	// }
```

Patch 5: Use assert for debugging in development builds
```
< 	simple_recursive_removal(dir, NULL);
---
> 	assert(dir != NULL);
> 	simple_recursive_removal(dir, NULL);
```

Note: The assert patch should only be used in development builds and not in production code, as it will terminate the program if the assertion fails.