xprtrdma: Fix a BUG when tracing is enabled with NFSv4.1 on RDMA

A backchannel reply does not set task->tk_client.

Fixes: 0c77668d ("SUNRPC: Introduce trace points in ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/trace/events/rpcrdma.h

@@ -559,7 +559,8 @@ TRACE_EVENT(xprtrdma_post_send,
 		const struct rpc_rqst *rqst = &req->rl_slot;
 
 		__entry->task_id = rqst->rq_task->tk_pid;
-		__entry->client_id = rqst->rq_task->tk_client->cl_clid;
+		__entry->client_id = rqst->rq_task->tk_client ?
+				     rqst->rq_task->tk_client->cl_clid : -1;
 		__entry->req = req;
 		__entry->num_sge = req->rl_sendctx->sc_wr.num_sge;
 		__entry->signaled = req->rl_sendctx->sc_wr.send_flags &