Fast Clone Deletion

Deleting a clone requires finding blocks are clone-only, not shared
with the snapshot. This was done by traversing the entire block tree
which results in a large performance penalty for sparsely
written clones.

This is new method keeps track of clone blocks when they are
modified in a "Livelist" so that, when it’s time to delete,
the clone-specific blocks are already at hand.

We see performance improvements because now deletion work is
proportional to the number of clone-modified blocks, not the size
of the original dataset.

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Sara Hartse <sara.hartse@delphix.com>
Closes #8416

include/sys/dsl_pool.h

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2013, 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2018 by Delphix. All rights reserved.
  * Copyright 2016 Nexenta Systems, Inc.  All rights reserved.
  */
 
@@ -54,6 +54,7 @@ struct dsl_pool;
 struct dmu_tx;
 struct dsl_scan;
 struct dsl_crypto_params;
+struct dsl_deadlist;
 
 extern unsigned long zfs_dirty_data_max;
 extern unsigned long zfs_dirty_data_max_max;