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: Sašo 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@c3a66015199e1814c2af8f7e6929af2004302e7e

Ported by: Cyril Plisko <cyril.plisko@mountall.com>

Porting note:

This port uses kstat_t instead of "struct kstat". Both are the same
in Illumos. However Linux has its own (unrelated) "struct kstat",
so SPL introduces "struct kstat_s". kstat_t should be cleaner in this
context anyhow.