MFV r323913: 8600 ZFS channel programs - snapshot

illumos/illumos-gate@2840dce1a029098fb784afd951d5f98089f850d8
https://github.com/illumos/illumos-gate/commit/2840dce1a029098fb784afd951d5f98089f850d8

https://www.illumos.org/issues/8600

ZFS channel programs should be able to create snapshots.
In addition to the base snapshot functionality, this will likely entail adding
extra logic to handle edge cases which were formerly not possible, such as
creating then destroying a snapshot in the same transaction sync.

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

MFC after: 5 weeks
X-MFC after: r324163