bug fix: pass the rkey of the mr to the client for rdma read/write operations instead of the lkey

[Old-Farmer]

In 'man ibv_reg_mr':

RETURN VALUE
ibv_reg_mr() / ibv_reg_mr_iova() / ibv_reg_dmabuf_mr() returns a pointer to the registered MR, or NULL if the request
fails. The local key (L_Key) field lkey is used as the lkey field of struct ibv_sge when posting buffers with
ibv_post_* verbs, and the the remote key (R_Key) field rkey is used by remote processes to perform Atomic and RDMA operations. The remote process places this rkey as the rkey field of struct ibv_send_wr passed to the ibv_post_send
function.

So the rkey of mr should be passed to client for rdma operations.

The most interesting thing is that in most of implementaions, rkey and lkey of one mr is exactly the same. So the previous code usually runs correctly.