Illumos #3537

3537 want pool io kstats

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Sa?o Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Brendan Gregg <brendan.gregg@joyent.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:
  http://www.illumos.org/issues/3537
  illumos/illumos-gate@c3a6601

Ported by: Cyril Plisko <cyril.plisko@mountall.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

Porting Notes:

1. The patch was restructured to take advantage of the existing
   spa statistics infrastructure.  To accomplish this the kstat
   was moved in to spa->io_stats and the init/destroy code moved
   to spa_stats.c.

2. The I/O kstat was simply named <pool> which conflicted with the
   pool directory we had already created.  Therefore it was renamed
   to <pool>/io

3. An update handler was added to allow the kstat to be zeroed.

include/sys/spa.h

@@ -550,6 +550,7 @@ typedef struct spa_stats {
 	spa_stats_history_t	read_history;
 	spa_stats_history_t	txg_history;
 	spa_stats_history_t	tx_assign_histogram;
+	spa_stats_history_t	io_history;
 } spa_stats_t;
 
 typedef enum txg_state {