NTB: correct memory barrier

mmiowb is not sufficient to flush the data and is causing data
corruption.  Change to wmb and the data corruption is no more.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/ntb/ntb_transport.c

@@ -1009,7 +1009,7 @@ static void ntb_tx_copy_task(struct ntb_transport_qp *qp,
 	hdr->ver = qp->tx_pkts;
 
 	/* Ensure that the data is fully copied out before setting the flag */
-	mmiowb();
+	wmb();
 	hdr->flags = entry->flags | DESC_DONE_FLAG;
 
 	ntb_ring_sdb(qp->ndev, qp->qp_num);