Free memory_block pool

[Jan Hubicka]

> On 11/20/19 4:03 PM, Jan Hubicka wrote:
> > I have noticed that for Firefox around 1GB of peak memory use goes into
> > the fact that we never free memory_block_pool::freelist.
> 
> Great, can you reduce the peak memory by the 1GB with the patch?

About half of it, approx 0.5GB (8.3GB -> 7.8GB) judging from memory use
graphs.  I was checking and malloc returns to system everything over
128k (at least it claims so) and malloc_trim does not make much
difference.  I am not sure how the vmstat reacts to MADV_DONTNEED.

Honza