IB/mlx4: Correct max_srq_wr returned from mlx4_ib_query_device()

We need to keep a spare entry in the SRQ so that there always is a
next WQE available when posting receives (so that we can tell the
difference between a full queue and an empty queue).  So subtract 1
from the value HW gives us before reporting the limit on SRQ entries
to consumers.

Found by Mellanox QA.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/main.c

@@ -120,7 +120,7 @@ static int mlx4_ib_query_device(struct ib_device *ibdev,
 	props->max_qp_init_rd_atom = dev->dev->caps.max_qp_init_rdma;
 	props->max_res_rd_atom	   = props->max_qp_rd_atom * props->max_qp;
 	props->max_srq		   = dev->dev->caps.num_srqs - dev->dev->caps.reserved_srqs;
-	props->max_srq_wr	   = dev->dev->caps.max_srq_wqes;
+	props->max_srq_wr	   = dev->dev->caps.max_srq_wqes - 1;
 	props->max_srq_sge	   = dev->dev->caps.max_srq_sge;
 	props->local_ca_ack_delay  = dev->dev->caps.local_ca_ack_delay;
 	props->atomic_cap	   = dev->dev->caps.flags & MLX4_DEV_CAP_FLAG_ATOMIC ?