ZoL PR_8453
Author: Tom Caputi <tcaputi@datto.com>
Date:   Fri Mar 15 17:14:31 2019 -0400

    Multiple DVA Scrubbing Fix

    Currently, there is an issue in the sequential scrub code which
    prevents self healing from working in some cases. The scrub code
    will split up all DVA copies of a bp and issue each of them
    separately. The problem is that, since each of the DVAs is no
    longer associated with the others, the self healing code doesn't
    have the opportunity to repair problems that show up in one of the
    DVAs with the data from the others.

    This patch fixes this issue by ensuring that all IOs issued by the
    sequential scrub code include all DVAs. Initially, only the first
    DVA of each is attempted. If an issue arises, the IO is retried
    with all available copies, giving the self healing code a chance
    to correct the issue.

    To test this change, this patch also adds the ability for zinject
    to specify individual DVAs to inject read errors into. We then
    add a new test case that utilizes this functionality to ensure
    scrubs and self-healing reads can handle and transparently fix
    issues with individual copies of blocks.

This update is followup on #10405
While attempting to port this update, the following ZoL updates are included:

    551905dd4 vdev_mirror: kstat observables for preferred vdev
    d6c6590c5 vdev_mirror: load balancing fixes
    9f500936c FreeBSD r256956: Improve ZFS N-way mirror read performance by