Don't use dmu_buf_is_dirty() for unassigned transaction.

The dmu_buf_is_dirty() call doesn't make sense here for two reasons:

1. txg is 0 for unassigned tx, so it was a no-op.
2. It is equivalent of checking if we have dirty records and we are doing this few lines earlier.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #14825