IB/mlx4: Lock SQ lock in mlx4_ib_post_send()

Because of a typo, mlx4_ib_post_send() takes the same lock rq.lock as
mlx4_ib_post_recv().  Correct the code so the intended sq.lock is
taken when posting a send.

Noticed by Yossi Leybovitch and pointed out by Jack Morgenstein from
Mellanox.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/qp.c

@@ -1282,7 +1282,7 @@ int mlx4_ib_post_send(struct ib_qp *ibqp, struct ib_send_wr *wr,
 	int size;
 	int i;
 
-	spin_lock_irqsave(&qp->rq.lock, flags);
+	spin_lock_irqsave(&qp->sq.lock, flags);
 
 	ind = qp->sq.head;
 
@@ -1448,7 +1448,7 @@ out:
 			       (qp->sq.wqe_cnt - 1));
 	}
 
-	spin_unlock_irqrestore(&qp->rq.lock, flags);
+	spin_unlock_irqrestore(&qp->sq.lock, flags);
 
 	return err;
 }