RDMA/cma: Use PTR_RET rather than if (IS_ERR(...)) + PTR_ERR

Suggested by scripts/coccinelle/api/ptr_ret.cocci.
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/core/cma.c

@@ -3064,10 +3064,7 @@ static int cma_join_ib_multicast(struct rdma_id_private *id_priv,
 						id_priv->id.port_num, &rec,
 						comp_mask, GFP_KERNEL,
 						cma_ib_mc_handler, mc);
-	if (IS_ERR(mc->multicast.ib))
-		return PTR_ERR(mc->multicast.ib);
-
-	return 0;
+	return PTR_RET(mc->multicast.ib);
 }
 
 static void iboe_mcast_work_handler(struct work_struct *work)