5909 ensure that shared snap names don't become too long after promotion
Author: Andriy Gapon <avg@freebsd.org>
Reviewed by: George Wilson <george@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
illumos/illumos-gate@cb5842f8b0caaad0ed53535bd77042e933fdbafe