3996 want a libzfs_core API to rollback to latest snapshot
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Andy Stormont <andyjstormont@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
	[illumos-gate merge]

commit a7027df17fad220a20367b9d1eb251bc6300d203
    3996 want a libzfs_core API to rollback to latest snapshot