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/bplist.h

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018 by Delphix. All rights reserved.
  */
 
 #ifndef	_SYS_BPLIST_H
@@ -49,6 +50,7 @@ void bplist_destroy(bplist_t *bpl);
 void bplist_append(bplist_t *bpl, const blkptr_t *bp);
 void bplist_iterate(bplist_t *bpl, bplist_itor_t *func,
     void *arg, dmu_tx_t *tx);
+void bplist_clear(bplist_t *bpl);
 
 #ifdef	__cplusplus
 }