Mutex leak in dsl_dataset_hold_obj()

In addition to dsl_dataset_evict_async() releasing a hold, there is
an error case in dsl_dataset_hold_obj() which had missed 4 additional
release calls. This was introduced in a1d477c24.

openzfsonosx-commit: https://github.com/openzfsonosx/zfs/commit/63ff7f1c

Authored by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8517