Add ddt, ddt_entry, and l2arc_hdr caches

Back the allocations for ddt tables+entries and l2arc headers with
kmem caches.  This will reduce the cost of allocating these commonly
used structures and allow for greater visibility of them through the
/proc/spl/kmem/slab interface.

Signed-off-by: John Layman <jlayman@sagecloud.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1893

module/zfs/spa_misc.c

@@ -1653,6 +1653,7 @@ spa_init(int mode)
 	refcount_init();
 	unique_init();
 	space_map_init();
+	ddt_init();
 	zio_init();
 	dmu_init();
 	zil_init();
@@ -1675,6 +1676,7 @@ spa_fini(void)
 	zil_fini();
 	dmu_fini();
 	zio_fini();
+	ddt_fini();
 	space_map_fini();
 	unique_fini();
 	refcount_fini();