Fix: sunrpc: null rpc_clnt dereference in rpc_task_queued tracepoint

Based on upstream Linux commit:

commit 0be283f676a1e7b208db0c992283197ef8b52158
Author: Benjamin Coddington <bcodding@redhat.com>
Date:   Tue Jan 23 09:32:35 2018 -0500

    SUNRPC: Fix null rpc_clnt dereference in rpc_task_queued tracepoint

    Backchannel tasks will not have a reference to the rpc_clnt.  Return -1 for
    cl_clid in that case.

    Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
    Signed-off-by: Trond Myklebust <trondmy@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>