Fix: sunrpc instrumentation for 4.17

Fix: sunrpc instrumentation for 4.17

See upstream commit:

  commit e671edb9428c8a61662aaf8c39f5edced7cc45c7
  Author: Chuck Lever <chuck.lever@oracle.com>
  Date:   Fri Mar 16 10:33:44 2018 -0400

    sunrpc: Simplify synopsis of some trace points

    Clean up: struct rpc_task carries a pointer to a struct rpc_clnt,
    and in fact task->tk_client is always what is passed into trace
    points that are already passing @task.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>