svcrdma: Create a generic tracing class for displaying xdr_buf layout

This class can be used to create trace points in either the RPC
client or RPC server paths. It simply displays the length of each
part of an xdr_buf, which is useful to determine that the transport
and XDR codecs are operating correctly.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 7324b21..07992d3 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -2509,6 +2509,7 @@ call_decode(struct rpc_task *task)
 		goto out;
 
 	req->rq_rcv_buf.len = req->rq_private_buf.len;
+	trace_xprt_recvfrom(&req->rq_rcv_buf);
 
 	/* Check that the softirq receive buffer is valid */
 	WARN_ON(memcmp(&req->rq_rcv_buf, &req->rq_private_buf,