gnetworkmonitornetlink: Close the socket after disconnecting its GSources
gnetworkmonitornetlink: Close the socket after disconnecting its GSources

`read_netlink_messages()` is the callback attached to the netlink socket
(G_IO_IN). It calls `g_socket_receive_message()`. There is a race
condition that if the socket is closed while there is a pending call, we
will try to receive on a closed socket, which fails.

To avoid this, we switch the order of the operations around: first
destroy the source and then close the socket.

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