Description of problem:
    In rpc layer, we will notify RPC_TRANSPORT_MAP_XID_REQUEST event to rpc_clnt layer for getting original rpc_req when we get a reply. The info->rsp will be copyed from saved_frame.rsp in function rpc_clnt_fill_request_info.
    But the struct saved_frame's member rsp hasn't ever been set in submitting procedure which will cause request_info->rsp.payload_count==0 in function __socket_read_reply, and then it will get a new rsp_iobuf for current vectored reply, the rsp_iobuf has been set in function client3_3_readv actually.
    We don't need to get iobuf twice in one readv operation.

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