8502 illumos#7955 broke delegated datasets when libshare is not present

illumos/illumos-gate@1c18e8fbd8
1c18e8fbd8

https://www.illumos.org/issues/8502
  The code in lib/libzfs/common/libzfs_mount.c already basically handles
  the case when libshare is not installed. We just need to not fail in
  zfs_init_libshare_impl.  I tested this in lx and things work as
  expected. I also tested there trying to set sharenfs and sharesmb on
  the delegated dataset. Neither is allowed from within a zone.  The
  spew of msgs from a native zone is not ZFS specific. I see the same
  spew simply running the share command.

Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Yuri Pankov <yuripv@gmx.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Jerry Jelinek <jerry.jelinek@joyent.com>

lib/libzfs/common/libzfs_mount.c

@@ -24,6 +24,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2014, 2016 by Delphix. All rights reserved.
  * Copyright 2016 Igor Kozhukhov <ikozhukhov@gmail.com>
+ * Copyright 2017 Joyent, Inc.
  * Copyright 2017 RackTop Systems.
  */
 
@@ -664,8 +665,14 @@ _zfs_init_libshare(void)
 static int
 zfs_init_libshare_impl(libzfs_handle_t *zhandle, int service, void *arg)
 {
+	/*
+	 * libshare is either not installed or we're in a branded zone. The
+	 * rest of the wrapper functions around the libshare calls already
+	 * handle NULL function pointers, but we don't want the callers of
+	 * zfs_init_libshare() to fail prematurely if libshare is not available.
+	 */
 	if (_sa_init == NULL)
-		return (SA_CONFIG_ERR);
+		return (SA_OK);
 
 	/*
 	 * Attempt to refresh libshare. This is necessary if there was a cache