OpenZFS 7199 - dsl_dataset_rollback_sync may try to free already free blocks

7200 no blocks must be born in a txg after a snaphot is created
Authored by: Andriy Gapon <andriy.gapon@clusterhq.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7199
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/bfaed0b
Closes #5817