libata: zero xfer length on ATAPI data xfer IRQ is HSM violation

Treat zero xfer length as HSM violation.  While at it, add
unlikely()'s to ATAPI ireason and transfer length checks.

tj: Formatted patch and added unlikely()'s.
Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -5309,12 +5309,15 @@ static void atapi_pio_bytes(struct ata_queued_cmd *qc)
 	bytes = (bc_hi << 8) | bc_lo;
 
 	/* shall be cleared to zero, indicating xfer of data */
-	if (ireason & (1 << 0))
+	if (unlikely(ireason & (1 << 0)))
 		goto err_out;
 
 	/* make sure transfer direction matches expected */
 	i_write = ((ireason & (1 << 1)) == 0) ? 1 : 0;
-	if (do_write != i_write)
+	if (unlikely(do_write != i_write))
+		goto err_out;
+
+	if (unlikely(!bytes))
 		goto err_out;
 
 	VPRINTK("ata%u: xfering %d bytes\n", ap->print_id, bytes);