Pool allocation classes

Allocation Classes add the ability to have allocation classes in a
pool that are dedicated to serving specific block categories, such
as DDT data, metadata, and small file blocks. A pool can opt-in to
this feature by adding a 'special' or 'dedup' top-level VDEV.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Reviewed-by: Håkan Johansson <f96hajo@chalmers.se>
Reviewed-by: Andreas Dilger <andreas.dilger@chamcloud.com>
Reviewed-by: DHE <git@dehacked.net>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Gregor Kopka <gregor@kopka.net>
Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #5182

include/sys/zio.h

@@ -353,6 +353,7 @@ typedef struct zio_prop {
 	uint8_t			zp_salt[ZIO_DATA_SALT_LEN];
 	uint8_t			zp_iv[ZIO_DATA_IV_LEN];
 	uint8_t			zp_mac[ZIO_DATA_MAC_LEN];
+	uint32_t		zp_zpl_smallblk;
 } zio_prop_t;
 
 typedef struct zio_cksum_report zio_cksum_report_t;
@@ -473,6 +474,7 @@ struct zio {
 	vdev_t		*io_vd;
 	void		*io_vsd;
 	const zio_vsd_ops_t *io_vsd_ops;
+	metaslab_class_t *io_metaslab_class;	/* dva throttle class */
 
 	uint64_t	io_offset;
 	hrtime_t	io_timestamp;	/* submitted at */