Illumos 5515 - dataset user hold doesn't reject empty tags

5515 dataset user hold doesn't reject empty tags
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

References:

https://www.illumos.org/issues/5515
https://github.com/illumos/illumos-gate/commit/752fd8d

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>