1700 Add SCSI UNMAP support

illumos/illumos-gate@047c81d31d0f571d6652e97143cd15281de61e84

Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Saso Kiselkov <saso.kiselkov@nexenta.com>