libnvdimm, pmem: flush posted-write queues on shutdown

Commit writes to media on system shutdown or pmem driver unload.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/nd.h

@@ -26,6 +26,7 @@ struct nd_device_driver {
 	unsigned long type;
 	int (*probe)(struct device *dev);
 	int (*remove)(struct device *dev);
+	void (*shutdown)(struct device *dev);
 	void (*notify)(struct device *dev, enum nvdimm_event event);
 };