Linpool flush drops all the allocated pages but one

Linpool flush drops all the allocated pages but one

When a linpool is used to allocate a one-off big load of memory, it
makes no sense to keep that amount of memory for future use inside the
linpool. Contrary to previous implementations where the memory was
directly free()d, we now use the page allocator which has an internal
cache which keeps the released pages for us and subsequent allocations
simply get these released pages back.

And even if the page cleanup routine kicks in inbetween, the pages get
only madvise()d, not munmap()ed so performance aspects are negligible.

This may fix some memory usage peaks in extreme cases.