RTS memory return behaviour is incorrect for non-moving GC

In !10381 (comment 498455): @teo says:

@mpickering apologies for not catching this before this got merged, but I don't think this is right for the case when gen is oldest_gen and we are using the non-moving gc. Then oldest_gen->n_blocks should be counted along with uncopied things as it's the blocks that make up segments I think