libata: fix bug with non-ncq devices

The recent commit 2fca5ccf ("libata:
switch to using block layer tagging support") to enable support for
block layer tagging in libata was broken for non-NCQ devices

The block layer initializes the tag field to -1 to detect invalid uses
of a tag, and if the libata devices does NOT support NCQ, we just used
that field to index the internal command list.  So we need to check for
-1 first and only use the tag field if it's valid.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Tested-by: Paul Mundt <lethal@linux-sh.org>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
Tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/ata/libata-scsi.c

@@ -708,7 +708,11 @@ static struct ata_queued_cmd *ata_scsi_qc_new(struct ata_device *dev,
 {
 	struct ata_queued_cmd *qc;
 
+	if (cmd->request->tag != -1)
 		qc = ata_qc_new_init(dev, cmd->request->tag);
+	else
+		qc = ata_qc_new_init(dev, 0);
+
 	if (qc) {
 		qc->scsicmd = cmd;
 		qc->scsidone = done;