sd-dhcp-client: add FIXME comment about the state callback

At least currently, it is a theoretical concern, as networkd does not
change the client state in the callback.

src/libsystemd-network/sd-dhcp-client.c

@@ -629,6 +629,8 @@ static void client_set_state(sd_dhcp_client *client, DHCPState state) {
                 ;
         }
 
+        // FIXME: If the state callback changes the state, we may not safely free/stop the client, and the
+        // state machine diagram becomes needlessly complicated. Introduce a guard to avoid that. */
         if (client->state_callback)
                 client->state_callback(client, state, client->state_userdata);
 }