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/txg.c

@@ -524,6 +524,8 @@ txg_delay(dsl_pool_t *dp, uint64_t txg, int ticks)
 		(void) cv_timedwait(&tx->tx_quiesce_more_cv, &tx->tx_sync_lock,
 		    timeout);
 
+	DMU_TX_STAT_BUMP(dmu_tx_delay);
+
 	mutex_exit(&tx->tx_sync_lock);
 }