Impossible --netfilter-mode=on mode set on Synology

### What is the issue?

I tried to run `tailscale up --advertise-connector` on my device, which led to the following amusing output:

```
grove@t6:~$ tailscale up --advertise-connector
Error: changing settings via 'tailscale up' requires mentioning all
non-default flags. To proceed, either re-run your command with --reset or
use the command below to explicitly mention the current value of
all non-default settings:

tailscale up --advertise-connector --advertise-exit-node --netfilter-mode=on

grove@t6:~$ tailscale up --advertise-connector --advertise-exit-node --netfilter-mode=on
--netfilter-mode values besides "off" not supported on Synology; see https://github.com/tailscale/tailscale/issues/1995

grove@t6:~$ tailscale up --advertise-connector --advertise-exit-node
Error: changing settings via 'tailscale up' requires mentioning all
non-default flags. To proceed, either re-run your command with --reset or
use the command below to explicitly mention the current value of
all non-default settings:

tailscale up --advertise-connector --advertise-exit-node --netfilter-mode=on

grovex@t6:~$
```

### Steps to reproduce

_No response_

### Are there any recent changes that introduced the issue?

_No response_

### OS

Synology

### OS version

Synology DSM7

### Tailscale version

1.92.3

### Other software

_No response_

### Bug report

BUG-04778a9a0d77bc2bb42dbf4ad74176ccedd5a66ccb0c1b6057f71f9115dcf6b1-20260303201231Z-23004c980dc595c4