libata: initialize qc->dma_dir to DMA_NONE

libata didn't used to init qc->dma_dir to any specific value on qc
initialization and command translation path didn't set qc->dma_dir if
the command doesn't need data transfer.  This made non-data commands
to have random qc->dma_dir.

This usually doesn't cause problem because LLDs usually check
qc->protocol first and look at qc->dma_dir iff the command needs data
transfer but this doesn't hold for all LLDs.

It might be worthwhile to rename qc->dma_dir to qc->data_dir as we use
the field to tag data direction for both PIO and DMA protocols.

This problem has been spotted by James Bottomley.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

include/linux/libata.h

@@ -1143,6 +1143,7 @@ static inline void ata_tf_init(struct ata_device *dev, struct ata_taskfile *tf)
 
 static inline void ata_qc_reinit(struct ata_queued_cmd *qc)
 {
+	qc->dma_dir = DMA_NONE;
 	qc->__sg = NULL;
 	qc->flags = 0;
 	qc->cursect = qc->cursg = qc->cursg_ofs = 0;