Illumos 5909 - ensure that shared snap names don't become too long after promotion

Illumos 5909 - ensure that shared snap names don't become too long after promotion

5909 ensure that shared snap names don't become too long after promotion
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/5909
  https://github.com/illumos/illumos-gate/commit/cb5842f

Ported-by: Andriy Gapon <avg@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3550