Illumos #3329, #3330, #3331, #3335

3329 spa_sync() spends 10-20% of its time in spa_free_sync_cb()
3330 space_seg_t should have its own kmem_cache
3331 deferred frees should happen after sync_pass 1
3335 make SYNC_PASS_* constants tunable

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Dan McDonald <danmcd@nexenta.com>
Approved by: Eric Schrock <eric.schrock@delphix.com>

References:
  illumos/illumos-gate@01f55e48fb
  https://www.illumos.org/issues/3329
  https://www.illumos.org/issues/3330
  https://www.illumos.org/issues/3331
  https://www.illumos.org/issues/3335

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/spa_misc.c

@@ -1633,6 +1633,7 @@ spa_init(int mode)
 	fm_init();
 	refcount_init();
 	unique_init();
+	space_map_init();
 	zio_init();
 	dmu_init();
 	zil_init();
@@ -1655,6 +1656,7 @@ spa_fini(void)
 	zil_fini();
 	dmu_fini();
 	zio_fini();
+	space_map_fini();
 	unique_fini();
 	refcount_fini();
 	fm_fini();