Illumos #3086: unnecessarily setting DS_FLAG_INCONSISTENT on async

3086 unnecessarily setting DS_FLAG_INCONSISTENT on async
destroyed datasets
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:

illumos/illumos-gate@ce636f8b38e8c9ff484e880d9abb27251a882860
illumos changeset: 13776:cd512c80fd75
https://www.illumos.org/issues/3086

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>