Illumos #1092: zfs refratio property

Add a "REFRATIO" property, which is the compression ratio based on
data referenced. For snapshots, this is the same as COMPRESSRATIO,
but for filesystems/volumes, the COMPRESSRATIO is based on the
data "USED" (ie, includes blocks in children, but not blocks
shared with the origin).

This is needed to figure out how much space a filesystem would
use if it were not compressed (ignoring snapshots).

Reviewed by: George Wilson <George.Wilson@delphix.com>
Reviewed by: Adam Leventhal <Adam.Leventhal@delphix.com>
Reviewed by: Dan McDonald <danmcd@nexenta.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Reviewed by: Mark Musante <Mark.Musante@oracle.com>
Reviewed by: Garrett D'Amore <garrett@nexenta.com>
Approved by: Garrett D'Amore <garrett@nexenta.com>

References to Illumos issue and patch:
- https://www.illumos.org/issues/1092
- https://github.com/illumos/illumos-gate/commit/187d6ac08a

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #340

lib/libzfs/libzfs_dataset.c

@@ -21,6 +21,7 @@
 
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2011 by Delphix. All rights reserved.
  */
 
 #include <ctype.h>
@@ -2025,6 +2026,7 @@ zfs_prop_get(zfs_handle_t *zhp, zfs_prop_t prop, char *propbuf, size_t proplen,
 		}
 		break;
 
+	case ZFS_PROP_REFRATIO:
 	case ZFS_PROP_COMPRESSRATIO:
 		if (get_numeric_property(zhp, prop, src, &source, &val) != 0)
 			return (-1);