nvme-rdma: introduce NVME_RDMA_MAX_METADATA_QUEUE_SIZE definition

This definition will be used by controllers that are configured with
metadata support. For now, both regular and metadata controllers have
the same maximal queue size but later commit will increase the maximal
queue size for regular RDMA controllers to 256.
We'll keep the maximal queue size for metadata controllers to be 128
since there are more resources that are needed for metadata operations
and 128 is the optimal size found for metadata controllers base on
testing.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>