8592 ZFS channel programs - rollback

illumos/illumos-gate@000cce6b6fad4a8b0eecef6e1251f6aca1719c55
https://github.com/illumos/illumos-gate/commit/000cce6b6fad4a8b0eecef6e1251f6aca1719c55

https://www.illumos.org/issues/8592

ZFS channel programs should be able to perform a rollback. This logic will
probably look pretty similar to zfs.sync.destroy().

Reviewed by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Brad Lewis <brad.lewis@delphix.com>