integrity: support detached data device.

Since the kernel 4.18 there is a possibility to speficy external
data device for dm-integrity that stores all integrity tags.

The new option --data-device in integritysetup uses this feature.

lib/utils_dm.h

@@ -122,6 +122,8 @@ struct crypt_dm_active_device {
 
 		const char *journal_crypt;
 		struct volume_key *journal_crypt_key;
+
+		struct device *meta_device;
 	} integrity;
 	} u;
 };