Enhance a dmu_tx_dirty_buf() assertion

The following assertion is good to validate the correctness of
new DMU consumers, but it doesn't quite provide enough information.
Slightly rework the assertion so that when it is hit the actual
offending values will be included in the output.

SPLError: 4787:0:(dmu_tx.c:828:dmu_tx_dirty_buf())
ASSERTION(dn == NULL || dn->dn_assigned_txg == tx->tx_txg) failed

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