OpenZFS 7571 - non-present readonly numeric ZFS props do not have def…

…ault value

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7571
OpenZFS-commit: openzfs/openzfs@ad2760a
Closes #5638

lib/libzfs/libzfs_dataset.c

@@ -22,7 +22,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
- * Copyright (c) 2011, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2016 by Delphix. All rights reserved.
  * Copyright (c) 2012 DEY Storage Systems, Inc.  All rights reserved.
  * Copyright (c) 2012 Pawel Jakub Dawidek <pawel@dawidek.net>.
  * Copyright (c) 2013 Martin Matuska. All rights reserved.
@@ -2109,6 +2109,7 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 			    !zfs_prop_setonce(prop) &&
 			    *source != NULL && (*source)[0] == '\0') {
 				*source = NULL;
+				return (-1);
 			}
 			break;