Uncommit space on memory mapped heaps

Since kernel 2.6.38 Linux has the capability to "uncommit" space in memory mapped files (FALLOC_FL_PUNCH_HOLE, http://man7.org/linux/man-pages/man2/fallocate.2.html).

This would allow collectors mapping the Java heap from a file (via e.g. AllocateHeapAt) for e.g. providing Java heap on NVDIMM uncommit unused space.

ZGC will also use this facility planned with JEP 351: ZGC: Uncommit Unused Memory, however it is useful for all collectors.