Merge miscellaneous libnvdimm updates for 4.21

* Use common helpers, bitmap_zalloc() and kstrndup(), to replace open
  coded versions.
* Clarify the comments around hotplug vs initial init case for the nfit
  driver.
* Cleanup the libnvdimm init path.

drivers/acpi/nfit/core.c

@@ -1331,7 +1331,7 @@ static ssize_t scrub_store(struct device *dev,
 	if (nd_desc) {
 		struct acpi_nfit_desc *acpi_desc = to_acpi_desc(nd_desc);
 
-		rc = acpi_nfit_ars_rescan(acpi_desc, 0);
+		rc = acpi_nfit_ars_rescan(acpi_desc, ARS_REQ_LONG);
 	}
 	device_unlock(dev);
 	if (rc)
@@ -3535,7 +3535,13 @@ static int acpi_nfit_add(struct acpi_device *adev)
 
 	status = acpi_get_table(ACPI_SIG_NFIT, 0, &tbl);
 	if (ACPI_FAILURE(status)) {
-		/* This is ok, we could have an nvdimm hotplugged later */
+		/* The NVDIMM root device allows OS to trigger enumeration of
+		 * NVDIMMs through NFIT at boot time and re-enumeration at
+		 * root level via the _FIT method during runtime.
+		 * This is ok to return 0 here, we could have an nvdimm
+		 * hotplugged later and evaluate _FIT method which returns
+		 * data in the format of a series of NFIT Structures.
+		 */
 		dev_dbg(dev, "failed to find NFIT at startup\n");
 		return 0;
 	}