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

module/zfs/dsl_pool.c

@@ -721,7 +721,8 @@ dsl_pool_sync(dsl_pool_t *dp, uint64_t txg)
 	 * Now that the datasets have been completely synced, we can
 	 * clean up our in-memory structures accumulated while syncing:
 	 *
-	 *  - move dead blocks from the pending deadlist to the on-disk deadlist
+	 *  - move dead blocks from the pending deadlist and livelists
+	 *    to the on-disk versions
 	 *  - release hold from dsl_dataset_dirty()
 	 *  - release key mapping hold from dsl_dataset_dirty()
 	 */