networkd: manager do not unef netlink and gennetlink early

networkd: manager do not unef netlink and gennetlink early

Because of this the fd is getting closed and we getting errors
like
```
^Ceno1: Could not send rtnetlink message: Bad file descriptor
enp7s0f0: Could not send rtnetlink message: Bad file descriptor
enp7s0f0: Cannot delete unreachable route for DHCPv6 delegated subnet 2a0a:...:fc::/62: Bad file descriptor
Assertion '*_head == _item' failed at ../systemd/src/network/networkd-route.c:126, function route_free(). Aborting.
Aborted
```

Closes one of https://github.com/systemd/systemd/issues/12452