nfit, libnvdimm: async region scrub workqueue

Introduce a workqueue that will be used to run address range scrub
asynchronously with the rest of nvdimm device probing.

Userspace still wants notification when probing operations complete, so
introduce a new callback to flush this workqueue when userspace is
awaiting probe completion.

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

include/linux/libnvdimm.h

@@ -71,6 +71,7 @@ struct nvdimm_bus_descriptor {
 	unsigned long dsm_mask;
 	char *provider_name;
 	ndctl_fn ndctl;
+	int (*flush_probe)(struct nvdimm_bus_descriptor *nd_desc);
 };
 
 struct nd_cmd_desc {