networkd: Don't try to close fd in sd_radv_stop if fd is closed.

networkd: Don't try to close fd in sd_radv_stop if fd is closed.

sd_radv_stop is called from two places. if sd_radv_stop is alrady
success then just don't try to close it .
```
systemd-networkd[604]: RADV: Stopping IPv6 Router Advertisement daemon
systemd-networkd[604]: RADV: Unable to send last Router Advertisement with router lifetime set to zero: Bad file descriptor <==================HERE
systemd-networkd[604]: RADV: Updated prefix 2a0a:*:*:fc::/64 preferred 1h valid 2h
systemd-networkd[604]: RADV: Started IPv6 Router Advertisement daemon
```

Closes one of the issue #8960