libnvdimm: Update name of the ars_status_record mask field

The spec suggests that this is a simple 'length' field, not a mask.
Update the name accordingly.
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/uapi/linux/ndctl.h

@@ -87,7 +87,7 @@ struct nd_cmd_ars_status {
 		__u32 handle;
 		__u32 flags;
 		__u64 err_address;
-		__u64 mask;
+		__u64 length;
 	} __packed records[0];
 } __packed;