nvmet-rdma: implement get_max_queue_size controller op

Limit the maximal queue size for RDMA controllers. Today, the target
reports a limit of 1024 and this limit isn't valid for some of the RDMA
based controllers. For now, limit RDMA transport to 128 entries (the
max queue depth configured for Linux NVMe/RDMA host).

Future general solution should use RDMA/core API to calculate this size
according to device capabilities and number of WRs needed per NVMe IO
request.

Reported-by: Mark Ruijter <mruijter@primelogic.nl>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>