RDS/IB: get the xmit max_sge from the RDS IB device on the connection

rds_ib_xmit_rdma() was calling ib_get_client_data() to get at the rds_ibdevice
just to get the max_sge for the transmit.  This patch instead has it get it
directly off the rds_ibdev which is stored on the connection.

The current code won't free the rds_ibdev until all the IB connections that use
it are freed.  So it's safe to reference the rds_ibdev this way.  In the future
it also makes it easier to support proper reference counting of the rds_ibdev
struct.

As an additional bonus, this gets rid of the performance hit of calling in to
the IB stack to look up the rds_ibdev.  The current implementation in the IB
stack acquires an interrupt blocking spinlock to protect the registration of
client callback data.
Signed-off-by: Zach Brown <zach.brown@oracle.com>

net/rds/ib_send.c

@@ -806,10 +806,10 @@ int rds_ib_xmit_rdma(struct rds_connection *conn, struct rm_rdma_op *op)
 	struct rds_ib_send_work *first;
 	struct rds_ib_send_work *prev;
 	struct ib_send_wr *failed_wr;
-	struct rds_ib_device *rds_ibdev;
 	struct scatterlist *scat;
 	unsigned long len;
 	u64 remote_addr = op->op_remote_addr;
+	u32 max_sge = ic->rds_ibdev->max_sge;
 	u32 pos;
 	u32 work_alloc;
 	u32 i;
@@ -818,8 +818,6 @@ int rds_ib_xmit_rdma(struct rds_connection *conn, struct rm_rdma_op *op)
 	int ret;
 	int num_sge;
 
-	rds_ibdev = ib_get_client_data(ic->i_cm_id->device, &rds_ib_client);
-
 	/* map the op the first time we see it */
 	if (!op->op_mapped) {
 		op->op_count = ib_dma_map_sg(ic->i_cm_id->device,
@@ -839,7 +837,7 @@ int rds_ib_xmit_rdma(struct rds_connection *conn, struct rm_rdma_op *op)
 	 * Instead of knowing how to return a partial rdma read/write we insist that there
 	 * be enough work requests to send the entire message.
 	 */
-	i = ceil(op->op_count, rds_ibdev->max_sge);
+	i = ceil(op->op_count, max_sge);
 
 	work_alloc = rds_ib_ring_alloc(&ic->i_send_ring, i, &pos);
 	if (work_alloc != i) {
@@ -867,9 +865,9 @@ int rds_ib_xmit_rdma(struct rds_connection *conn, struct rm_rdma_op *op)
 		send->s_wr.wr.rdma.remote_addr = remote_addr;
 		send->s_wr.wr.rdma.rkey = op->op_rkey;
 
-		if (num_sge > rds_ibdev->max_sge) {
-			send->s_wr.num_sge = rds_ibdev->max_sge;
-			num_sge -= rds_ibdev->max_sge;
+		if (num_sge > max_sge) {
+			send->s_wr.num_sge = max_sge;
+			num_sge -= max_sge;
 		} else {
 			send->s_wr.num_sge = num_sge;
 		}