libata: fix bug with non-ncq devices

The recent commit 2fca5ccf97d2c28bcfce44f5b07d85e74e3cd18e ("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 <[e-mail]>
Reported-by: Alexander Beregalov <[e-mail]>
Tested-by: Paul Mundt <[e-mail]>
Tested-by: Dave Young <[e-mail]>
Tested-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/ata/libata-scsi.c

···
 {
 	struct ata_queued_cmd *qc;
 
-	qc = ata_qc_new_init(dev, cmd->request->tag);
+	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;