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.com


Signed-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>