Add 'dmu_tx' kstats entry

Keep counters for the various reasons that a thread may end up
in txg_wait_open() waiting on a new txg.  This can be useful
when attempting to determine why a particular workload is
under performing.

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

module/zfs/dsl_dataset.c

@@ -3290,6 +3290,8 @@ dsl_dataset_check_quota(dsl_dataset_t *ds, boolean_t check_quota,
 			error = ERESTART;
 		else
 			error = EDQUOT;
+
+		DMU_TX_STAT_BUMP(dmu_tx_quota);
 	}
 	mutex_exit(&ds->ds_lock);