dsl_dataset_rollback_sync may try to free already freed blocks when it calls
dsl_destroy_head_sync_impl to destroy a temporary clone.
That happens if a snapshot to which we are rolling back and from which the
clone is created has some ZIL records.

No new blocks must be born in a dataset in the same TXG after a snapshot of the
dataset is taken.
Those blocks would have the same blk_birth as the dataset's ds_prev_snap_txg
and as such they would be presumed to belong o the snapshot while in fact they
do not.
All the datasets must be clean before sync tasks are run, so the described
scenario may happen only if one of the sync tasks dirties the dataset and
another sync task takes its snapshot.
Then, there will be another sync pass because of the dirty data and the new
blocks will be born in the same TXG when the data is written out.
It seems that almost all of the existing sync tasks modify only MOS and do not
dirty any objsets.
The only exception that I've been able to identify so far is the rollback which
can modify an objset when it zeroes out the objset's ZIL.