Fix mmap() usage in ztest.

Fix mmap() usage in ztest.

illumos/illumos-gate@ad135b5d644628e791c3188a6ecbd9c257961ef8
Illumos changeset: 13700:2889e2596bd6

Note that this is only a partial port of the aforementioned Illumos
changeset.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <gwilson@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Dan Kruchinin <dan.kruchinin@gmail.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

Ported to zfsonlinux by: Etienne Dechamps <etienne.dechamps@ovh.net>