Use SET_ERROR for constant non-zero return codes

Update many return and assignment statements to follow the convention
of using the SET_ERROR macro when returning a hard-coded non-zero
value from a function. This aids debugging by recording the error
codes in the debug log.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #6441

module/zfs/dmu_tx.c

@@ -888,7 +888,7 @@ dmu_tx_try_assign(dmu_tx_t *tx, txg_how_t txg_how)
 	    dsl_pool_need_dirty_delay(tx->tx_pool)) {
 		tx->tx_wait_dirty = B_TRUE;
 		DMU_TX_STAT_BUMP(dmu_tx_dirty_delay);
-		return (ERESTART);
+		return (SET_ERROR(ERESTART));
 	}
 
 	tx->tx_txg = txg_hold_open(tx->tx_pool, &tx->tx_txgh);