libata: fix ata_scmd_need_defer()

Fix ata_scmd_need_defer() such that...

* whether NCQ is used or not is exactly determined using the same
  criteria as the issue path.

* defer-check is performed in all cases.

This fixes race condition where turning off NCQ on the fly causes
non-NCQ commands sneak into NCQ phase.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/ata/libata-scsi.c

@@ -1496,11 +1496,9 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
 static int ata_scmd_need_defer(struct ata_device *dev, int is_io)
 {
 	struct ata_port *ap = dev->ap;
+	int is_ncq = is_io && ata_ncq_enabled(dev);
 
-	if (!(dev->flags & ATA_DFLAG_NCQ))
-		return 0;
-
-	if (is_io) {
+	if (is_ncq) {
 		if (!ata_tag_valid(ap->active_tag))
 			return 0;
 	} else {