nvme: implement the Flush command Implement a real flush instead of faking it. This is especially important as Qemu assume Write back cashing by default and thus requires a working cache flush operation for data integrity. Signed-off-by: Christoph Hellwig <hch@lst.de> Acked-by: Keith Busch <keith.busch@intel.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>