8856 arc_cksum_is_equal() doesn't take into account ABD-logic

illumos/illumos-gate@01a059ee0cdece49f47fd4d70086dd5bc7d0b0ff

https://www.illumos.org/issues/8856:
arc_cksum_is_equal() calls zio_push_transform() that requires abd_t*
(second arg), but a void* is passed.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Roman Strashkin <roman.strashkin@nexenta.com>