Add createtxg sort support for simple snapshot iterator

- When iterating snapshots with name only, e.g., "-o name -s name",
libzfs uses simple snapshot iterator and results are displayed
in alphabetic order. This PR adds support for faster version of
createtxg sort by avoiding nvlist parsing for properties. Flags
"-o name -s createtxg" will enable createtxg sort while using
simple snapshot iterator.
- Added support to read createtxg property directly from zfs handle
for filesystem, volume and snapshot types instead of parsing nvlist.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #13577

module/zfs/zfs_ioctl.c

@@ -2322,6 +2322,8 @@ zfs_ioc_snapshot_list_next(zfs_cmd_t *zc)
 		}
 
 		if (zc->zc_simple) {
+			zc->zc_objset_stats.dds_creation_txg =
+			    dsl_get_creationtxg(ds);
 			dsl_dataset_rele(ds, FTAG);
 			break;
 		}