SUNRPC: Fix oops in the rpc_xdr_buf event class

Backchannel rpc tasks don't have task->tk_client set, so it's necessary
to check it for NULL before dereferencing.

Fixes: c509f15a ("SUNRPC: Split the xdr_buf event class")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/trace/events/sunrpc.h

@@ -68,7 +68,8 @@ DECLARE_EVENT_CLASS(rpc_xdr_buf_class,
 
 	TP_fast_assign(
 		__entry->task_id = task->tk_pid;
-		__entry->client_id = task->tk_client->cl_clid;
+		__entry->client_id = task->tk_client ?
+				     task->tk_client->cl_clid : -1;
 		__entry->head_base = xdr->head[0].iov_base;
 		__entry->head_len = xdr->head[0].iov_len;
 		__entry->tail_base = xdr->tail[0].iov_base;