Deregisters a memory buffer that had been registered for RDMA
or message operations. A user should call rdma_dereg_mr for all
registered memory associated with an rdma_cm_id before destroying
Returns 0 on success, or -1 on error. If an error occurs, errno will be
set to indicate the failure reason.
All memory registered with an rdma_cm_id is associated with the
protection domain associated with the id. Users must deregister
all registered memory before the protection domain can be destroyed.