Illumos #3740

3740 Poor ZFS send / receive performance due to snapshot
     hold / release processing
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:
  https://www.illumos.org/issues/3740
  illumos/illumos-gate@a7a845e4bf

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775

Porting notes:

1. 13fe019870 introduced a merge conflict
   in dsl_dataset_user_release_tmp where some variables were moved
   outside of the preprocessor directive.

2. dea9dfefdd747534b3846845629d2200f0616dad made the previous merge
   conflict worse by switching KM_SLEEP to KM_PUSHPAGE. This is notable
   because this commit refactors the code, adding a new KM_SLEEP
   allocation. It is not clear to me whether this should be converted
   to KM_PUSHPAGE.

3. We had a merge conflict in libzfs_sendrecv.c because of copyright
   notices.

4. Several small C99 compatibility fixed were made.

cmd/ztest/ztest.c

@@ -22,6 +22,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2012 by Delphix. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright (c) 2013 Steven Hartland. All rights reserved.
  */
 
 /*
@@ -4830,7 +4831,7 @@ ztest_dmu_snapshot_hold(ztest_ds_t *zd, uint64_t id)
 
 	error = user_release_one(fullname, tag);
 	if (error)
-		fatal(0, "user_release_one(%s)", fullname, tag);
+		fatal(0, "user_release_one(%s, %s) = %d", fullname, tag, error);
 
 	VERIFY3U(dmu_objset_hold(fullname, FTAG, &origin), ==, ENOENT);