7604 if volblocksize property is the default, it displays as "-" rath…

…er than 8K

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/lib/libzfs/common/libzfs_dataset.c

@@ -2125,9 +2125,12 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 			/*
 			 * If we tried to use a default value for a
 			 * readonly property, it means that it was not
-			 * present.
+			 * present.  Note this only applies to "truly"
+			 * readonly properties, not set-once properties
+			 * like volblocksize.
 			 */
 			if (zfs_prop_readonly(prop) &&
+			    !zfs_prop_setonce(prop) &&
 			    *source != NULL && (*source)[0] == '\0') {
 				*source = NULL;
 				return (-1);