vmem: add vmem_malloc_usable_size() to API

I think we could be more restrictive here. But this would require the information about size classes that jemalloc is using (see: http://www.canonware.com/download/jemalloc/jemalloc-latest/doc/jemalloc.html#size_classes).
I.e. if *size* is in (512..1024] range, then *usable_size - size <= 128*
Probably we could include some header file from jemalloc or calculate size classes and their spacing in run time, based on current chunk size.