networkd: immediately transfer ownership of route->section

networkd: immediately transfer ownership of route->section

The code was not incorrect previously, but I think it's easier to follow the
ownership (and the code is more likely to remain correct when updated later on),
if freeing of NetworkConfigSection* is immediately made the responsibility of
route_free(), so instead of relying on route_free() not freeing ->section
if adding to the network hashmap failed, make this freeing unconditional.