Enable scatter ABD in userspace

This patch enables ztest to use scatter ABD.

Part of this patch comes from:
https://github.com/DeHackEd/zfs/commit/845369df411827dfc095acddf788fc26540cae55

Suggested-by: DHE <git@dehacked.net>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>