IB/mthca: memfree completion with error FW bug workaround

Memfree firmware is in rare cases reporting WQE index == base - 1 in
receive completion with error, instead of (rq size - 1); base is 0 in
mthca.  Here is a patch to avoid kernel crash and report a correct WR
id in this case.
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_cq.c

@@ -540,8 +540,17 @@ static inline int mthca_poll_one(struct mthca_dev *dev,
 		entry->wr_id = srq->wrid[wqe_index];
 		mthca_free_srq_wqe(srq, wqe);
 	} else {
+		s32 wqe;
 		wq = &(*cur_qp)->rq;
-		wqe_index = be32_to_cpu(cqe->wqe) >> wq->wqe_shift;
+		wqe = be32_to_cpu(cqe->wqe);
+		wqe_index = wqe >> wq->wqe_shift;
+               /*
+		* WQE addr == base - 1 might be reported in receive completion
+		* with error instead of (rq size - 1) by Sinai FW 1.0.800 and
+		* Arbel FW 5.1.400.  This bug should be fixed in later FW revs.
+		*/
+		if (unlikely(wqe_index < 0))
+			wqe_index = wq->max - 1;
 		entry->wr_id = (*cur_qp)->wrid[wqe_index];
 	}