was actually obtained from the buddy allocator.  This should avoid some
internal memory fragmentation in mem_heap_create() and mem_heap_alloc().

mem_area_alloc(): Change the in parameter size to an in/out parameter.
Adjust the size based on what was obtained from pool->free_list[].

mem_heap_create_block(): Adjust block->len to what was obtained from
mem_area_alloc().