OpenZFS 9166 - zfs storage pool checkpoint

Details about the motivation of this feature and its usage can
be found in this blogpost:

    https://sdimitro.github.io/post/zpool-checkpoint/

A lightning talk of this feature can be found here:
https://www.youtube.com/watch?v=fPQA8K40jAM

Implementation details can be found in big block comment of
spa_checkpoint.c

Side-changes that are relevant to this commit but not explained
elsewhere:

* renames members of "struct metaslab trees to be shorter without
  losing meaning

* space_map_{alloc,truncate}() accept a block size as a
  parameter. The reason is that in the current state all space
  maps that we allocate through the DMU use a global tunable
  (space_map_blksz) which defauls to 4KB. This is ok for metaslab
  space maps in terms of bandwirdth since they are scattered all
  over the disk. But for other space maps this default is probably
  not what we want. Examples are device removal's vdev_obsolete_sm
  or vdev_chedkpoint_sm from this review. Both of these have a
  1:1 relationship with each vdev and could benefit from a bigger
  block size.

Porting notes:

* The part of dsl_scan_sync() which handles async destroys has
  been moved into the new dsl_process_async_destroys() function.

* Remove "VERIFY(!(flags & FWRITE))" in "kernel.c" so zhack can write
  to block device backed pools.

* ZTS:
  * Fix get_txg() in zpool_sync_001_pos due to "checkpoint_txg".

  * Don't use large dd block sizes on /dev/urandom under Linux in
    checkpoint_capacity.

  * Adopt Delphix-OS's setting of 4 (spa_asize_inflation =
    SPA_DVAS_PER_BP + 1) for the checkpoint_capacity test to speed
    its attempts to fill the pool

  * Create the base and nested pools with sync=disabled to speed up
    the "setup" phase.

  * Clear labels in test pool between checkpoint tests to avoid
    duplicate pool issues.

  * The import_rewind_device_replaced test has been marked as "known
    to fail" for the reasons listed in its DISCLAIMER.

  * New module parameters:

      zfs_spa_discard_memory_limit,
      zfs_remove_max_bytes_pause (not documented - debugging only)
      vdev_max_ms_count (formerly metaslabs_per_vdev)
      vdev_min_ms_count

Authored by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Tim Chase <tim@chase2k.com>

OpenZFS-issue: https://illumos.org/issues/9166
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7159fdb8
Closes #7570

include/sys/space_map.h

@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2012, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
  */
 
 #ifndef _SYS_SPACE_MAP_H
@@ -57,7 +57,7 @@ extern "C" {
 typedef struct space_map_phys {
 	uint64_t	smp_object;	/* on-disk space map object */
 	uint64_t	smp_objsize;	/* size of the object */
-	uint64_t	smp_alloc;	/* space allocated from the map */
+	int64_t		smp_alloc;	/* space allocated from the map */
 	uint64_t	smp_pad[5];	/* reserved */
 
 	/*
@@ -82,7 +82,7 @@ typedef struct space_map {
 	uint64_t	sm_size;	/* size of map */
 	uint8_t		sm_shift;	/* unit shift */
 	uint64_t	sm_length;	/* synced length */
-	uint64_t	sm_alloc;	/* synced space allocated */
+	int64_t		sm_alloc;	/* synced space allocated */
 	objset_t	*sm_os;		/* objset for this map */
 	uint64_t	sm_object;	/* object id for this map */
 	uint32_t	sm_blksz;	/* block size for space map */
@@ -140,6 +140,8 @@ typedef int (*sm_cb_t)(maptype_t type, uint64_t offset, uint64_t size,
 
 int space_map_load(space_map_t *sm, range_tree_t *rt, maptype_t maptype);
 int space_map_iterate(space_map_t *sm, sm_cb_t callback, void *arg);
+int space_map_incremental_destroy(space_map_t *sm, sm_cb_t callback, void *arg,
+    dmu_tx_t *tx);
 
 void space_map_histogram_clear(space_map_t *sm);
 void space_map_histogram_add(space_map_t *sm, range_tree_t *rt,
@@ -153,8 +155,8 @@ uint64_t space_map_length(space_map_t *sm);
 
 void space_map_write(space_map_t *sm, range_tree_t *rt, maptype_t maptype,
     dmu_tx_t *tx);
-void space_map_truncate(space_map_t *sm, dmu_tx_t *tx);
-uint64_t space_map_alloc(objset_t *os, dmu_tx_t *tx);
+void space_map_truncate(space_map_t *sm, int blocksize, dmu_tx_t *tx);
+uint64_t space_map_alloc(objset_t *os, int blocksize, dmu_tx_t *tx);
 void space_map_free(space_map_t *sm, dmu_tx_t *tx);
 void space_map_free_obj(objset_t *os, uint64_t smobj, dmu_tx_t *tx);