network/dhcp4: send release message before stopping the client

network/dhcp4: send release message before stopping the client

Otherwise, the socket is already closed and sending release will be
anyway skipped.

With this patch, release message is sent before stopping the client.
```
Jan 20 18:29:41 systemd[1]: Stopping systemd-networkd.service - Network Management...
Jan 20 18:29:41 systemd-networkd[3821255]: wlp59s0: DHCPv4 client: RELEASE
Jan 20 18:29:41 systemd-networkd[3821255]: wlp59s0: DHCPv4 client: STOPPED
Jan 20 18:29:41 systemd-networkd[3821255]: wlp59s0: DHCP lease lost
```

Fixes #39299.