The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice is:

The code does not check if the 'nn' pointer returned by net_generic() is NULL before dereferencing it in the rpc_proc_register() call. If net_generic() fails and returns NULL, attempting to access nn->rpcstats will result in a NULL pointer dereference.

To fix this vulnerability, a NULL check should be added after obtaining the 'nn' pointer:

```c
struct nfs_net *nn = net_generic(net, nfs_net_id);
if (nn == NULL) {
    // Handle error condition
    return -ENOMEM; // Or appropriate error code
}
rpc_proc_register(net, &nn->rpcstats);
```

This ensures that 'nn' is valid before attempting to access its members, preventing the potential NULL pointer dereference.