Based on the provided code slice and the identified vulnerability (CWE-476: NULL Pointer Dereference) on line 6, here are five possible patches to address the issue:

Patch 1: Add NULL check for 'nn' before dereferencing
```
6     if (nn) {
6a        rpc_proc_register(net, &nn->rpcstats);
6b    }
```

Patch 2: Add NULL check for 'nn' and return an error if NULL
```
6     if (!nn) {
6a        return -ENOMEM;
6b    }
6c    rpc_proc_register(net, &nn->rpcstats);
```

Patch 3: Use a temporary variable with NULL check
```
6     struct rpc_stat *stats = nn ? &nn->rpcstats : NULL;
6a    if (stats) {
6b        rpc_proc_register(net, stats);
6c    }
```

Patch 4: Initialize 'nn' to a default value if NULL
```
3     struct nfs_net *nn = net_generic(net, nfs_net_id);
3a    if (!nn) {
3b        nn = &default_nfs_net;
3c    }
```

Patch 5: Add error handling for net_generic and return early
```
3     struct nfs_net *nn = net_generic(net, nfs_net_id);
3a    if (!nn) {
3b        pr_err("Failed to allocate nfs_net\n");
3c        return -ENOMEM;
3d    }
```

These patches aim to prevent the NULL pointer dereference by adding various checks and error handling mechanisms. The choice of patch depends on the specific requirements and error handling strategy of the project.