OpenZFS 4521 - zfstest is trying to execute evil "zfs unmount -a"

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Porting Notes:
- Correctly set __ZFS_POOL_RESTRICT in inherit_001_pos

OpenZFS-issue: https://www.illumos.org/issues/4521
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8808ac5
Closes #5674

cmd/zfs/zfs_main.c

@@ -25,8 +25,8 @@
  * Copyright 2012 Milan Jurik. All rights reserved.
  * Copyright (c) 2012, Joyent, Inc. All rights reserved.
  * Copyright (c) 2013 Steven Hartland.  All rights reserved.
- * Copyright 2013 Nexenta Systems, Inc.  All rights reserved.
  * Copyright 2016 Igor Kozhukhov <ikozhukhov@gmail.com>.
+ * Copyright 2016 Nexenta Systems, Inc.
  */
 
 #include <assert.h>
@@ -6563,6 +6563,15 @@ unshare_unmount(int op, int argc, char **argv)
 				continue;
 			}
 
+			/*
+			 * Ignore datasets that are excluded/restricted by
+			 * parent pool name.
+			 */
+			if (zpool_skip_pool(zfs_get_pool_name(zhp))) {
+				zfs_close(zhp);
+				continue;
+			}
+
 			switch (op) {
 			case OP_SHARE:
 				verify(zfs_prop_get(zhp, ZFS_PROP_SHARENFS,