IB/core: Read subnet_prefix in ib_query_port via cache.

ib_query_port() calls device->ops.query_port() to get the port
attributes. The method of querying is device driver specific.  The same
function calls device->ops.query_gid() to get the GID and extract the
subnet_prefix (gid_prefix).

The GID and subnet_prefix are stored in a cache. But they do not get
read from the cache if the device is an Infiniband device. The
following change takes advantage of the cached subnet_prefix.
Testing with RDBMS has shown a significant improvement in performance
with this change.

Link: https://lore.kernel.org/r/20210712122625.1147-4-anand.a.khoje@oracle.comSigned-off-by: Anand Khoje <anand.a.khoje@oracle.com>
Signed-off-by: Haakon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/device.c

@@ -2052,7 +2052,6 @@ static int __ib_query_port(struct ib_device *device,
 			   u32 port_num,
 			   struct ib_port_attr *port_attr)
 {
-	union ib_gid gid = {};
 	int err;
 
 	memset(port_attr, 0, sizeof(*port_attr));
@@ -2065,11 +2064,8 @@ static int __ib_query_port(struct ib_device *device,
 	    IB_LINK_LAYER_INFINIBAND)
 		return 0;
 
-	err = device->ops.query_gid(device, port_num, 0, &gid);
-	if (err)
-		return err;
-
-	port_attr->subnet_prefix = be64_to_cpu(gid.global.subnet_prefix);
+	ib_get_cached_subnet_prefix(device, port_num,
+				    &port_attr->subnet_prefix);
 	return 0;
 }