SUNRPC: Move the task->tk_bytes_sent and tk_rtt to struct rpc_rqst

It seems strange to maintain stats for bytes_sent in one structure, and
bytes received in another. Try to assemble all the RPC request-related
stats in struct rpc_rqst

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>