LU-10174 osd-zfs: Fix build against ZFS 0.6.5.x

Ensure all values used in https://review.whamcloud.com/29242

 are
actually used conditionally.  This allows us to build against
ZFS 0.6.5.7 again.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ide560e21f7b7fedc0c107b1943602e5af99b478c
Reviewed-on: https://review.whamcloud.com/29866


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>

lustre/osd-zfs/osd_handler.c

@@ -778,13 +778,14 @@ static void osd_readonly_changed_cb(void *arg, uint64_t newval)
 	osd->od_prop_rdonly = !!newval;
 }
 
+#ifdef HAVE_DMU_OBJECT_ALLOC_DNSIZE
 static void osd_dnodesize_changed_cb(void *arg, uint64_t newval)
 {
 	struct osd_device *osd = arg;
 
 	osd->od_dnsize = newval;
 }
-
+#endif
 /*
  * This function unregisters all registered callbacks.  It's harmless to
  * unregister callbacks that were never registered so it is used to safely
@@ -800,8 +801,10 @@ static void osd_objset_unregister_callbacks(struct osd_device *o)
 				   osd_recordsize_changed_cb, o);
 	(void) dsl_prop_unregister(ds, zfs_prop_to_name(ZFS_PROP_READONLY),
 				   osd_readonly_changed_cb, o);
+#ifdef HAVE_DMU_OBJECT_ALLOC_DNSIZE
 	(void) dsl_prop_unregister(ds, zfs_prop_to_name(ZFS_PROP_DNODESIZE),
-				   osd_readonly_changed_cb, o);
+				   osd_dnodesize_changed_cb, o);
+#endif
 
 	if (o->arc_prune_cb != NULL) {
 		arc_remove_prune_callback(o->arc_prune_cb);
@@ -838,10 +841,12 @@ static int osd_objset_register_callbacks(struct osd_device *o)
 	if (rc)
 		GOTO(err, rc);
 
+#ifdef HAVE_DMU_OBJECT_ALLOC_DNSIZE
 	rc = -dsl_prop_register(ds, zfs_prop_to_name(ZFS_PROP_DNODESIZE),
 				osd_dnodesize_changed_cb, o);
 	if (rc)
 		GOTO(err, rc);
+#endif
 
 	o->arc_prune_cb = arc_add_prune_callback(arc_prune_func, o);
 err: