rdma_reg_read is used to register a data buffer that will be the
target of an RDMA read operation on a queue pair associated with
an rdma_cm_id. The memory buffer is registered with the proteection
domain associated with the idenfier. The start of the data buffer
is specified through the addr parameter, and the total size of the buffer
is given by length.
All data buffers should be registered before being posted as a work request.
Users must deregister all registered memory by calling rdma_dereg_mr.