RDMA/cxgb4: rmb() after reading valid gen bit

Some HW platforms can reorder read operations, so we must rmb() after
we see a valid gen bit in a CQE but before we read any other fields
from the CQE.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/cxgb4/t4.h

@@ -620,6 +620,9 @@ static inline int t4_next_hw_cqe(struct t4_cq *cq, struct t4_cqe **cqe)
 		printk(KERN_ERR MOD "cq overflow cqid %u\n", cq->cqid);
 		BUG_ON(1);
 	} else if (t4_valid_cqe(cq, &cq->queue[cq->cidx])) {
+
+		/* Ensure CQE is flushed to memory */
+		rmb();
 		*cqe = &cq->queue[cq->cidx];
 		ret = 0;
 	} else