Dramatically optimize listing snapshots when user requests only snapshot

names and wants to sort them by name, ie. when executes:

	# zfs list -t snapshot -o name -s name

Because only name is needed we don't have to read all snapshot properties.

Below you can find how long does it take to list 34509 snapshots from a single
disk pool before and after this change with cold and warm cache:

before:

	# time zfs list -t snapshot -o name -s name > /dev/null
	cold cache: 525s
	warm cache: 218s

after:

	# time zfs list -t snapshot -o name -s name > /dev/null
	cold cache: 1.7s
	warm cache: 1.1s

MFC after:	1 week

1 files changed, 3 insertions, 1 deletions

diff --git a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c
index b0f034b..1a305fd 100644
--- a/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c
+++ b/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c
@@ -22,6 +22,8 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011 by Delphix. All rights reserved.
+ * Copyright (c) 2012 Pawel Jakub Dawidek <pawel@dawidek.net>.
+ * All rights reserved.
  */
 
 #include <assert.h>
@@ -717,7 +719,7 @@ send_iterate_fs(zfs_handle_t *zhp, void *arg)
 	sd->parent_fromsnap_guid = 0;
 	VERIFY(0 == nvlist_alloc(&sd->parent_snaps, NV_UNIQUE_NAME, 0));
 	VERIFY(0 == nvlist_alloc(&sd->snapprops, NV_UNIQUE_NAME, 0));
-	(void) zfs_iter_snapshots(zhp, send_iterate_snap, sd);
+	(void) zfs_iter_snapshots(zhp, B_FALSE, send_iterate_snap, sd);
 	VERIFY(0 == nvlist_add_nvlist(nvfs, "snaps", sd->parent_snaps));
 	VERIFY(0 == nvlist_add_nvlist(nvfs, "snapprops", sd->snapprops));
 	nvlist_free(sd->parent_snaps);