MFC r322230: MFV r322229: 7600 zfs rollback should pass target snapshot to kernel

MFC r322230: MFV r322229: 7600 zfs rollback should pass target snapshot to kernel

illumos/illumos-gate@77b171372ed21642e04c873ef1e87fe2365520df
https://github.com/illumos/illumos-gate/commit/77b171372ed21642e04c873ef1e87fe2365520df

https://www.illumos.org/issues/7600
  At present, the kernel side code seems to blindly rollback to whatever happens
  to be the latest snapshot at the time when the rollback task is processed.
  The expected target's name should be passed to the kernel driver and the sync
  task should validate that the target exists and that it is the latest snapshot
  indeed.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>