mlx5 devices (Connect-IB, ConnectX-4, ConnectX-4-LX) has a limitation
where rdma read work queue entries cannot exceed 512 bytes.
A rdma_read wqe needs to fit in 512 bytes:
- wqe control segment (16 bytes)
- rdma segment (16 bytes)
- scatter elements (16 bytes each)

So max_sge_rd should be: (512 - 16 - 16) / 16 = 30.

Cc: linux-stable@vger.kernel.org
Reported-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSagi Grimberg <sagig@grimberg.me>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>